Priming - Vertical Stabilizer, Rudder and Horizontal Stabilizer
With some priming practice under my belt, it was time to prepare all the vertical stabilizer, rudder and horizontal stabilizer parts that I had been working on in recent weeks for priming. I started by cleaning the smaller parts with EkoEtch and laying them out to dry on my priming table.
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| Ribs and stiffeners etched and ready for priming |
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| Etching the skins outdoors |
I also took pictures of sharpie markings I had made on the spars for bracket orientation. This will help later during riveting as the EkoEtch will remove any markings on the aluminum.
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| Bracket alignment marks on vertical stabilizer spar |
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| More bracket alignment marks |
With all the parts cleaned and etched, I left them to dry thoroughly in preparation for priming the next day.
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| All parts etched and ready to prime |
Priming
Having set the spray gun during the priming practice a few days earlier, I proceeded to prime the airplane parts I had prepared. I initially mixed up a batch of 200g EkoPrime + 20g distilled water and started priming the skins. I primed the parts outside with a morning temperature of about 60F.
Priming started well although it felt as if the gun was spraying more liquid than when I had practiced earlier with the same settings. I turned down the liquid volume and proceeded to prime the vertical stabilizer and rudder skins. Prior to priming the rudder skins, I had also taped off the trailing edges as I didn't want to prime where the proseal would later hold the trailing edge wedge in place.
I did notice that the primer was taking a long time to dry on the skins. I believe this was due to the cooler outdoor temperature. Stewart Systems recommends priming in temperatures of 60F-85F. When I primed the skins it was barely 60F and as the morning got warmer (65F-70F) I saw that the primer dried faster.
With the skins primed, I then proceeded to prime the spars followed by the smaller parts. Here is where I started running into some issues while priming the vertical stabilizer and horizontal stabilizer spars. I first primed the inside of the spars, and soon noticed the primer started to run down the flanges in a few places. I tried to wipe the drips away using some scrap paper, but the edges of the drips had already thickened and started to dry. I turned down the liquid volume on the spray gun further and sprayed a second coat.
I believe the drips were caused by a combination of cooler temperature and too much liquid being sprayed. I tweaked the spray gun setting some more as the daytime temperature rose, and ultimately was able to finish spraying the remainder of the parts with good results. I also had to mix up an additional 300g of primer, for a total of 500g+50g used to finish priming all the parts. This seemed a bit much to me, and confirms my suspicion that too much liquid was initially being sprayed, resulting in a thicker than needed coat of primer.
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| Primed spars and spar caps. Note: I did not prime the trailing edge wedge |
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| Primed ribs and stiffeners |
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| Primed rudder skins, except for the trailing edge |
Following a short vacation away from home, I decided to try and fix the drip marks on the rear vertical stabilizer and rear horizontal stabilizer parts as those would be visible even after riveting.
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| Look closely and you will see drip marks on the vertical stabilizer spar |
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| More drip marks |
My options were to sand the primer from the spars, or try to remove it completely using acetone. My first thought was to try and remove the primer from the affected parts with acetone and reprime the surface as that would be sure to remove evidence of my amateurish priming skills.
However, removing the EkoPrime with a cloth soaked in acetone takes FOREVER, and also risks damaging the primer finish on the opposite side of the part where the acetone leaks through the holes. I therefore DO NOT recommend trying to fix primer mistakes this way. I ended up just removing a small section of primer this way before deciding it was not worth the effort. I finished by carefully cleaning the bare aluminum with a ScotchBrite pad sprayed with EkoEtch.
I then tried the second option of sanding the EkoPrime (using a maroon ScotchBrite pad) just enough the remove the raised edges of the drip marks. EkoPrime sands really well, even after a week of being left to cure.
With prep done, I proceeded to respray the EkoPrime (after tweaking the spray gun settings yet again) making sure that I was spraying thin coats of primer over 2-3 passes, and this time I was very happy with the results. Even over the sanded parts it is now hard to tell that there was a drip without looking closely, and I feel that if I need to fix any priming mistakes in the future, sanding is the way to go rather than trying to remove the primer completely.
Priming all these parts took longer than necessary, but I'm learning from my mistakes, and am confident that my priming skills will continue to improve.
Primer removed with acetone and reprimed
Priming Hints
- Spray EkoPrime when temperatures are 70-80F, otherwise it takes a long time to dry and increases the risk of drips forming on the primed surface
- Take time to adjust the spray gun to ensure that liquid volume being sprayed results in a thin coat of primer. Surfaces should be primed in 2-3 thin coats rather than a single thick coat. The first coat can be very thin (tack coat) to give the following coats something to stick to.
- After spraying one side, leave parts for 5-10 minutes before spraying the opposite sides to minimize the chance of runs
- Do not try to remove EkoPrime using solvents as it takes a lot of effort and a very long time. Instead, sand any mistakes away and reprime.
- Charcoal Gray is a good color for seeing coverage as primer is being applied, however, I think I will switch to Smoke Gray for the interior parts of the plane that will be visible as I prefer the lighter gray color.
Time Taken: 8.8 hours
Dates: July 22 - July 24 and August 1 - August 2 2022
RV-10 Build Total Time: 39.6 hours
Priming Total Time: 8.8 hours (not included in build time totals)