The following is a write-up provided by Doug G. His detailed narrative of priming is helpful for anyone in the research or production phase of building an aircraft, particularly those priming with Akzo Nobel, such as me.

As always, this should not be misconstrued as instruction, direction, or advice. I, nor Doug, accept any responsibility for harm,–physical, financial, or otherwise–from failure to adhere to manufacturer directions and basic safety precautions.

And last, thank you Doug, for your contribution to one of the most researched topics of plane construction. Couldn’t appreciate it more.

Priming the Empennage of my RV10

Preface

Because I knew that setting up to paint parts was going to be a major operation, I decided to approach the build a little different than most.  I completed each section of the build up to the point where priming was suggested prior to assembly and then moved on to the next section.  In this way, the entire Empennage was ready for priming at the same time and all priming would be completed with one setup.

Parts Preparation

After some research I decided to use Prekote with scotch brite pads to prep the parts for painting.  The instructions call for the parts to painted within 24 hours of preparation.  I initially thought I would prep all the parts and then paint but that was not possible so I adopted the procedure of prepping the parts during the day and then painting them in the evening.  In order to not waste paint, the plan was to have more parts prepped than I had paint to complete but that did not always work so at times I had some mixed paint carried over to the next day and this worked okay even though the pot life was only supposed to be 8 hours.  Note:  I did find that after the 2^nd day of sitting, the paint did not shake to the same consistency and I would not want to use it.

I cut the scotch brite pads into either 8 or 12 pieces depending on the size of the parts to be cleaned and this game me something easier to handle and I think I got better utilization of the pads.  I did find that they don’t last all that long and I would change pieces about every 10 minutes.  I would try to make sure I used both sides and folded them often to make it easier to hold.  I just used the nitrile gloves while cleaning and then I rinsed them with soft water.  I bought a box of painters rags from the Home Depot and used these to wipe the parts dry after rinsing.  The small parts I did in the laundry sink and the larger parts I put on a plastic folding table on the lawn beside my driveway and rinsed with the hose.  I did not want to do them directly on the driveway in case the grey soapy water that is rinsed off dried and left a stain.  I did not see any adverse effects of the rinse water on the lawn.

What do you do with all those sharpie marks on the parts that are going to be scrubbed off with the Prekote/scotch brite cleaning?  Where I could, I made a little sketch and noted the marks so I could put them back on after cleaning and drying.  In some cases, I used my phone to make a video of the part so I could play it back in stop motion to put the marks back on in the right places.  Be careful not to flip the parts around during the cleaning process or the marks will not go back in the right place.  Note:  I did see the marks thru the paint but they were hard to see in places (paint probably too thick in spots).  I really won’t know the extent of how well this worked until assembly.

I did all the dimpling prior to prepping for paint.  I did find it difficult to get a good scotch brite scrub around the dimples by just passing the pad over the line of dimples and this was also very aggressive on the scotch brite pad.   What worked best for me was to scrub right up parallel to the line of dimples on each side and then use a small piece of scotch brite and work it around each dimple and between the dimples.  This gave a good matte finish to the dimples and around them.  I thought this detail was important since these dimples (particularly in the skins) will be mating with other parts and you want a good paint bond in this joint.  This certainly adds to the tediousness of the job and my index finger is still a little numb nearly a week after finishing the job (but it is recovering). 

I also saw some using scotch brite on a palm sander to do the skins and I considered trying this but most were doing this dry and I was concerned about damaging my sander with the liquid.  In all honesty, the flat open areas were done rather easily – it was the areas around the dimples that took the time.  The procedure for Prekote is to apply 1 coat and scrub in one direction and then apply a second coat before the first dries and scrub in a second direction.  I believe this is to make sure nothing gets missed.  I only applied one coat and in larger areas, I used a circular motion to scrub but most of the small parts, you scrub in any way you can.  The matte surface after scrubbing shows it has been scrubbed.  I did find when working outside that you had to work in fairly small areas and rinse frequently or the material would start to dry.  I also noticed when drying with the rags that there would still be some black come off the parts on to the rag.  I wore gloves during the cleaning and drying process and never handled the parts with bare hands once the cleaning process started until after they were painted and the paint was dry to make sure no oil from my hands got on the parts.  Gloves was the only PPE used during prep.

Regarding the quantity of Prekote, I purchased the 1 qt spray bottle and the 1 gallon refill.  I was very liberal in wetting the parts and the scotch brite pads so by the time I was finished the empennage, I had about 1 qt left so I used about 1 gallon for the job.  Looking back, I’m sure I could have completed the job with about half that quantity if I tried to conserve.

Spray Booth

I am building my kit in the 2 car garage attached to our house in our subdivision with my neighbors very close so I must take the steps I can to minimize the negative effects of my work on their enjoyment of their property.  As examples, I insulated the garage including the door to reduce the noise escaping.  I also designed and built an effective noise enclosure for my air compressor which also makes it much nicer for me working in the garage.  When I considered spray painting the airplane parts, I certainly could not consider painting outside with the chance overspray getting on my house or car or that of my neighbors so I never considered painting without a spray booth.  With my limited workspace and the need to paint larger parts, I knew the spray both would have to take up my workspace when it was in place and that it needed to be able to be setup and taken down.  I considered different options but finally decided on a frame of 1×2 covered with 6 mil plastic.  I also wanted to be able to move parts in and out of the booth easily and at the recommendation of a painter friend, I decided on an open faced booth (3 walls, and a ceiling).  The booth had to fit under my garage door opener and the width had to fit between the equipment along both sides leaving a narrow walkway along one side an along the tool boxes.  As for the length, I decided on 8’ so the overall dimensions were 7’ high x 7’ wide x 8’ long.  I wanted to make the booth foldable so I could store it in my basement when not in use so the walls and ceiling were made of 2’ long x 7’ high frames which were hinged together (hinges made by cutting piano hinges into sections) so they folded like a room divider and the plastic was staples with enough slack at the hinges to allow the panels to fold.  This made the walls light enough to move and the frames were held in the open position with bolts thru the fame and each wall also bolted together.  To assemble the booth, a sheet of 6 mil poly is placed on the floor and the booth assembled on top.  To protect the plastic from getting covered with overspray, scrap cardboard is stapled to the inside of the booth half way up the walls and cardboard is also laid on the floor over the plastic in the booth as well as the area in front of the open face.  I also designed and built a drying rack with I position in front of the open face so I can place parts there waiting to paint as well as parts waiting to dry. 

I was very concerned about trapping the overspray so it did not get all over the outside of the house or impact the neighbors.  I had previously used a single box fan with furnace filters for spaying water based clear finish on wood projects but I did see some overspray escaping the open face and the filters did allow some of the finish to get thru to the fan so I knew I needed a better system for fan and filter.  I decided to buy regular paint booth filters (fiberglas 20” x 20” x 2” thick).  I wanted to reduce the velocity of the air thru the filters to allow better overspray capture so I decided to make the filter area in the wall opposite the open face 40” x 40” (not a calculated decision – best guess).  This worked well.  I also decided that I would make the filter opening deep enough to support 3 filters deep (assuming the air movement was not adversely affected).  I next turned my attention to the fan.  After doing some research, I found that an open faced booth should have an airflow of 100 ft/min so I estimated that I needed a fan that could produce approx 7000 cfm.  I found a fan at Home Depot that made this claim so I purchased it.  I then had to determine how I was going to made the filters and fan to the booth.  I decided to make a 40” x 40” x 18” deep plywood box open on one end to butt up the booth opening with weather stripping and the other end closed with a hole in the center to match the fan diameter.  I had to relocate the fan on/off switch from the face of the fan screen to the outside of the fan body to get it out of the air stream and make it accessible once the fan was attached to the box.  I also made plywood rings to attach the fan to the box.  In order to keep the friction fit filters from being pulled into the fan, I stapled chicken wire 6” back from the open face of the box.  I installed 1 layer of filters and checked the airflow (no measurement – just qualitative) which did not seen any less than with no filters.  I added a second layer of filters and the airflow still seemed nearly the same so I decided to start with 2 layers.  The fan box was complete and fastened to the opening in the booth wall opposite the open face with the fan just inside of the garage door.  When using the booth, the garage door is opened and the fan run on high speed.  As a precaution, I moved the car out of the driveway to the street.

Lighting was a concern since the front of the booth was under the garage door when the door was opened to use the booth and the ceiling lights were shielded by the door.  I had 4 spare 4’ LED fixtures for the ceiling lights so I plugged them one to the other and into an outlet I could turn on and off.  These fixtures were just set on top of the booth and provided good lighting.

The last booth concern was with the flammability of the paint/solvent.  I checked the flash point and by scanning organic solvents, it was the same as heptane so I used the data for this solvent for further consideration.  It was pointed out that in order for this material to burn, the mixture in air needed to be between 2 values (% by volume) – otherwise it would be too lean or too rich to burn.  I did a test with my spray gun set at a very high fluid output setting and measured how much material it output per minute.  When I calculated the %VOC by volume of air, it was orders of magnitude below the minimum required to burn – even if the fan was running at only 10% of stated output so I was satisfied that the mixture in the booth would be much too lean to burn.  I also did some research on the fan and determined that even though the fan motor is not explosion proof, it is an induction motor which by its nature does not produce sparks during regular operation.  Combining these, I was satisfied that fire should not be a concern with my setup.

PPE

The primer I was using for the airplane parts was the Akzo Nobel 463-12-8 primer (2 part epoxy) which requires good PPE and my spray equipment is a turbine unit with a separate turbine supplying air to my face mask.  I positioned the turbine unit outside the garage through the side door far from the fan exhaust.  I also had a shoot suit with booties, gloves and hood so I felt very safe from the paint effects.

Painting Equipment

I purchased a 4 stage turbine unit from Axis Citation which has 2 turbines contained in the unit – one for the paint gun and the second for fresh breathing air to the mask.  The turbine unit was placed on a mobile cart so that I could open the side door of the garage and move it outside which placed it around the corner and about 20’ away from the open garage door and booth exhaust fan (which was blowing away from the turbine unit).  I also purchased a remote switch for the paint gun turbine which allowed me to mount it on the wall of the booth together with a hook to hang the gun.  When I was ready to paint, I turned on the mask turbine at the unit and turned on the paint gun turbine with the remote switch inside the booth.  This worked well.  The mask has a very flexible short hose that reaches down to my waist with a quick connect to the mask hose coming from the turbine unit.  I have a belt with a clip to hold a ring on the mask air hose so it does not pull on the mask as you move around.  Having 2 hoses to handle in the booth can be a challenge at times but a little organization helps.  I did find that as I moved parts from the drying rack to the booth and back, I tended to always turn in the same direction which did cause some problems with this hose twisting.

The paint gun I have is an HVLP made by 3M (accuspray series 10) and it is equipped with the 3M PPS system – bottom feed.  There is a piece of silicone tubing that comes off the paint cup and fits on a nipple on the side of the gun.  When the gun is triggered, air from this nipple flows thru this tubing into the space between the paint cup and the liner causing the pressure created to squeeze the liner and pushing the paint up thru the lid filter into the gun to be sprayed.   I have 2 paint cups: 5 oz and 28 oz with liners and lids to match.  The spec sheet for the paint called for a 1.4 mm nozzle but the one I had was a 1.5 mm and this seemed to work well.  I also used a #8 air cap.

Painting Preparation

My intention was to strain the paint before putting it into the paint cup liner and I purchased strainers for this purpose along with popcicle sticks to stir but because the paint is so expensive, I wanted to come up with a handling system that minimized waste.  The lids of the paint cup liners have a very fine screen which filters the paint as it is being squeezed up into the gun so I decided to count on the lid screens and skip the pre filtering.  I purchased Mixing Mates from Rockler for each of the 2 one gallon cans (1 is the paint and the other is the hardener) and used these in place of the lids for the entire painting process.  They seemed to seal well as no smell was detected.  The Mixing Mate had a propeller mounted on a shaft coming out of the top of the lid and by turning the handle, you can mix the paint without the need for a stir stick (reducing waste).  The lid also has a sealed pouring spout that allows for dispensing the paint from the can with no spillage or need for dipping with a transfer cup – again reducing waste.  Each of the paint cups has a measuring scale film that slips between the cup and the liner so I used the 1:1 scale to dispense the paint directly from the can to the cup liner.  The spring loaded sliding seal over the spout on the Mixing Mate made it very easy to control the amount being added to the paint cup.  After adding the hardener up to the same number on the scale, I placed the lid on the liner and screwed on the lid of the paint cup.  I purchased plastic plugs with the lid/liners that fit the outlet of the lid and once you install the plug in the lid, it seals the lid/liner and contents.  To mix, I then just shook the paint cup vigorously for a minute or 2.  I purchased a gallon of thinner for this paint but did not add any thinner to the mix – I only used the thinner for cleanup.  I let the paint sit for 30 minutes minimum before using and I would usually give it another shake about every 10 minutes.  This paint does seem to separate even after sitting only a few minutes so it needs to be shaken again frequently – even during the painting session.  Once ready to paint, give it one last shake – remove the plug and attach it to the gun with a quarter twist and attach the paint gun cup to the nipple on the paint gun.  I used a respirator with organic cartridges whenever I had paint or thinner open unless I was wearing the fresh air mask of my painting equipment and I wore nitrile gloves except these did not stand up well to the thinner.  I did have one incident with the PPS system when a liner developed a leak and I didn’t discover it until I picked it up to shake it.  A little cleanup required but other than that, it worked very well.

Painting

The paint gun air hose has a quick connect at the gun and a quarter turn shutoff valve.  The turbine puts out a lot of air and I found in early painting sessions that I had good results and less overspray by throttling this air valve by about 30 degrees.  The paint gun has to adjustments.  The top knob is to adjust the width of the spray fan and the lower adjusts the amount of paint.  I found that the wider the fan, the more paint required and visa vera.  If you dialed the paint back too low for the fan setting, the paint would look dull as it was going on and it would have a sand paper feel when dry.  I found that for any setting, you wanted to see the paint shiny where it was overlapping the 50% so it was wet enough to level out and give a smooth finish.  That being said, the majority of the parts to paint are small or have small edges so again to minimize waste, you want to have the fan set quite narrow and adjust the paint so it just gives a shiny appearance on the second pass.  I found that with my gun, I did not have the paint knob opened more than ½ turn at any time and I had the best control with the fan set just above the minimum.  Even when painting skins,  I only widened the fan by a few degrees and the paint the same.  With a narrow fan, it takes more passes back and forth to complete the coverage but it worked well for me.  I also held the nozzle about 4 to 6” away from the part.  I found that if you got it much further away, it would still cover but you would have a rough texture (paint was drying as it was being applied – not wet and able to level).  I found this particularly on the Horizontal Stabilizer skins at the bend.  The paint was nice and smooth except as you got to the bend where it was like sand paper – not a real problem but might have to give it a light sanding where the nose ribs mate.

I built 2 tables about 2’ x 3’ with chicken wire on top to support small parts to paint but I only used it when I had to because I found it difficult to get around to all the edges.  I also built a turn table (piece of 3” plumbing drain pipe with toilet flanges on each end and a plywood disc attached for a base – 2 plywood discs on top with a lazy susan ring between to allow the top disc to spin).  I bought some plastic triangle standoffs to set on top of the lazy susan to hold the parts up off the plywood and this allowed me to spin the part to paint it instead of me having to move around the part.  This made it much easier to get to all the edges etc and I think I got a better job with this method.  This works particularly well for things like ribs.  I did find a problem with parts that were very light as the air from the gun would blow them off the stands so the tables worked good for these parts.  Some parts that were long and narrow, I would just hold in my hand and paint half of one surface and put it on the table to dry while completing the others.  Then, after about 10 minutes, the paint was dry enough that I could hold the painted end and finish painting the other end.  Then repeat the process for the other sides.  I had a challenge with the tail cone side skins as they were about 10’ long and my booth is only 8’.  I placed one table on each side of the booth and put one skin on one of the tables with the turn table supporting the other end.  I started painting the skin closest to the exhaust fan from side to side working my way away from the exhaust fan until I got half way down the skin.  Then I stopped and took the skin out of the booth and flipped it end for end and placed it on the table on the other side of the booth.  I was then able to resume painting where I just left off and worked my way back towards the exhaust fan to finish.  This worked well and I got a good finish by rolling my wrist as I got to the rolled edge so the gun stayed the same distance from the surface as it was on the flat.

I found that when the paint gun started running out of paint, you would see it as the paint stopped being shiny even with increasing the paint knob.  After removing the liner, there would be very little paint left in the liner but sometimes there would be some quantity and I was able to finish a few parts by holding the gun upside down to get gravity to help get the paint from the liner to the gun.

Regarding pot life, they say the paint will still be good for 8 hrs but my experience was that it was still good the next day.  I never used the paint mixed the previous day directly.  I would mix up the paint for today and add the left overs from yesterday to todays liner.  This never gave a problem.  I did not want to chance using the liner from the previous day in case the paint had cured the screen shut so I always disposed of the liner after pouring the contents into the new liner for today.

One thing I found was that as long as the paint is not exposed to the air, it doesn’t dry so what I would do when I had to stop for a few minutes is to put a piece of masking tape over the spray nozzle of the gun after wiping it with a little thinner on a rag just to keep it from drying out.  I also found that if I needed to mix up more paint in the paint cup, when I removed the paint cup, the tube leading to the paint gun from the cup was a perfect fit for the plastic plugs used to plug the lid of the liner.  In this way, you could remove the paint cup, insert the plug and the paint in the gun would not dry.  I left it this way for hours with no problems – just removed the plug – put the paint cup back on and resumed spraying – no gun cleanup required and no wasted paint!

Cleanup

With the PPS system, the paint cup liner/lid is the only thing wet with paint and when the paint gun runs out of paint, there is very little left in the liner.  Regardless of the quantity left over, I always kept it and added it to paint batch for the next day to keep the waste to a minimum.  After the painting was completed, I would remove the paint cup from the gun and detach the silicone tubing.  I had thinner in a squeeze bottle that I used to rinse out the paint tube leading from the paint cup to the gun and let this drain into a 1 gallon Home Depot bucket.  I then clipped on the second paint cup which had a liner filled with thinner and sprayed it thru the gun also into the HD bucket – spayed for about 10 seconds.  I then removed this paint cup and used the squeeze bottle with the trigger depressed to run thinner thru the inlet tube and out the nozzle.  I then removed the air cap and used a brush with a little thinner to give it a clean (usually not much on it).  I then removed the nozzle and used a small brush to run thru it – then rinsed with the wash bottle of thinner.  I then gave the needle/nozzle area a quick rinse with the wash bottle and reassembled.  I then hooked it back up the paint gun air supply and blew air thru the gun to clear the thinner – then hung it up for the next paint session.  I kept enough fresh thinner to be able to always have new thinner in the wash bottle but the thinner I spayed thru the gun, I reused after passing it thru a coffee filter.

Observations

I remember reading that the 2 gallon kit of primer should be enough paint to do the entire airplane.  That is not my experience.  I was getting concerned that I would run out before I finished priming the RV10 empennage but after finishing, I do have a little left over and I do mean a little.  I did all I could to reduce waste – I don’t think I threw away more than a couple ounces of mixed paint.  I tried to set the gun to minimize the overspray and I did video myself painting the elevator skins and it did not look like much was going into the air.  I look at the amount of overspray on the floor, tables etc and it does not look like a lot.  I will say the filter in the exhaust are certainly green so there was definitely overspray and it was pulled into the exhaust – difficult to tell how much that would be as a percentage.  I do believe I got a good coating on the parts but I don’t think it was excessive for the most part – just a wet coat.  That is what I observed in my operation for what its worth.

How much paint to mix up at one time?  I had many more 5 oz liners than the 28 oz liners so I used those the most.  I would often find that I would need to mix a second or a third for a painting session and this does require a 30 minute wait before spraying but that was often a welcomed break and allowed parts to dry well before flipping.  When it came to painting the skins which I left until last, I used the larger liners and found that mixing them about ½ full was enough to paint a good quantity but I only think I was able to get the quantity just right for the parts I had prepared once.

As far as the booth performance, I am very pleased.  After painting nearly 2 gallons of this primer, I can report that overspray was well contained.  Nothing was detected outside the open face and the outlet filters collected all but a trace on the fan blades.   The overspray dried so fast that most on the floor of the booth and in the filters is a dust.

So how long does it take to prime an RV10 empennage?  I did not keep track of hours – I never keep track of hours – it’s a hobby and time does not matter.  In this case, since the paint would only carry over for 1 day and the parts once prepared needed to be painted within 24 hours, this process was nearly a 7 day a week operation while running and I worked as steady as a retiree should work for a full 3 weeks to complete this job.  The prep is a lot of tedious elbow grease but you don’t want to do less than the best job you can since going back is not an option.

How did it turn out?  This is the first time I have ever sprayed paint.  All my previous experience has been with water based polyurethane finish on wood projects.  I did have some orange peel in a few spots and I did have some dry sand paper like finish in a few spots but overall, I am extremely pleased with the job I did.  It looks good and it feels good.

Would I recommend Priming?  It is peace of mind for me.

Would I recommend leaving the priming to all one session as I did?  It means that with all the hours I have spent on this process, the kit is still in pieces just as it arrived – well not quite as it arrived – but there is nothing finished to show anyone.  I am so looking forward to having nothing but assembly ahead but it has been a long road getting here.  I’m sure the comments now will be “well that really went together fast”.  I’m glad to have all the grunt work behind me for this kit but I think it would be better to complete pieces as you go if you can – just for the motivation factor.