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How to prep your cowling for paint

carrollcw

carrollcw

Well Known Member
So, I need to prepare my cowling for paint. I have tried the vans recommended method of mixing epoxy resin with acetone, but that is not working for me. Just gums up the sand paper. I have heard there is a good product to apply to the cowl to get a uniform finish, but cannot figure out what it is. Recommendations?
 
There is no one step process for filling the pinholes. I mixed up thick epoxy and micro slurry and squeegeed it into the pinholes with a single edge razor blade. Sand and repeat. Then I used the UV Smoothprime sprayed on several coats and sanded smooth. Repeat as needed.
 
Does smooth prime really need to dry for 3 weeks prior to prime and paint?!?!?! Any other options?
 
Cowling prep

when I was at a recent EAA meet a fellow giving a demo mixes wallboard paste and water into a glue consistency then squeegees it on. After that he sands and only dry wipes off the dust from sanding, primes over that and repeats if needed. The sanding dust stays in the pinholes and becomes hard when covered by primer. That's how I'm going to do mine when I get that far.
 
I had better luck wet sanding the epoxy skim coats......My main gear fairings had pinholes galore after shooting one coat of Stewart Systems Eko Prime. As an experiment, I dipped my plastic spreader in Eko Prime and then spread it on the fiberglass and worked it in back and forth in the holes. I eventually got all the holes filled in but there were some stubborn ones that I had to keep going back and forth over. The nice part about using primer like this is you can see the holes open back up if you don't get them filled. You can also sand in about an hour as opposed to waiting till the next day.

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I used smooth prime on my pants. Yes, yo do need to wait three weeks for any outgassing to stop. While doing my pants, I read danh's recommendation. I used epoxy, not diluted. Just make sure to get it as thin as possible. I did three coats. As long as you keep the epoxy thin, sanding isn't too bad. Results were m ugh nicer and quicker to obtain than using the smooth prime.
 
carrollcw said:
So, I need to prepare my cowling for paint. I have tried the vans recommended method of mixing epoxy resin with acetone, but that is not working for me. Just gums up the sand paper.
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At some risk of insulting the inhabitants of the Mother Ship, thinning epoxies at random with anything is really dumb....Russian roulette with chemistry.

Do some reading at the producer tech level. There are base epoxy resins thinned (by the original producer) with acetone, MEK, toulene, xylene, and other solvents. The solvents were not selected by spinning a bottle.

There are base epoxy resins that contain no solvents at all, and the producer did not intend that any be used....viscosity is adjusted by controlling the molecular weight of the finished product.

If you want a low-viscosity epoxy, just buy it. I've used lots of System Three Clear Coat, but there are others; just look at the viscosity spec.
 
Many years ago I used to do a lot of Corvette repair and restoration and we used a product called Featherfill to block the waves out of the fiberglass. It also worked very well for the everpresent pinholes. Could be described as a sprayable polyester filler (read bondo). Can be blocked with 80 grit or finer and can be topcoated with any quality primers. Haven't finished my cowling yet but I will try it when the time comes. Maybe someone on the forum has tried it on their epoxy parts and can give a report.
 
harleyl said:
Many years ago I used to do a lot of Corvette repair and restoration and we used a product called Featherfill to block the waves out of the fiberglass. It also worked very well for the everpresent pinholes. Could be described as a sprayable polyester filler (read bondo). Can be blocked with 80 grit or finer and can be topcoated with any quality primers. Haven't finished my cowling yet but I will try it when the time comes. Maybe someone on the forum has tried it on their epoxy parts and can give a report.
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That stuff is heavy. Be careful how much you apply, then sand off most of it.
 
Just gums up the sand paper.

In my limited experience using standard West 105/106 is too thick and leaves too thick of a covering layer and requires a lot of sanding which can produce an uneven surface.
Cutting it with acetone per Vans also produced a gummy soft covering that takes a week to harden up and it never gets completely cured.
If you have already done a bunch of your cowl that way, let it harden a while.

Then I found that coarser sandpaper, i.e. 150grit works better. Sand very lightly with a hard foam block and wipe the surface frequently with a lint free cloth to get the resulting "dust" off before it loads up the paper. I got an acceptable surface with very few pinholes left.

Dave A.
 
Kyle Boatright said:
That stuff is heavy. Be careful how much you apply, then sand off most of it.
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That's correct. Take it back down to the original surface and leave it in the low spots and pinholes. It generally leaves a good base that won't shrink. You can apply multiple coats but you need to really open your fluid control because it is thick.
 
koda2 said:
In my limited experience using standard West 105/106 is too thick and leaves too thick of a covering layer and requires a lot of sanding which can produce an uneven surface.
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Dave, that might be more application method than epoxy choice, and hardener choice can make a big difference too.

If you just paint West on, it will be pretty thick. Squeegee off all the excess, allow it to jell, squeegee on another coat, gell, repeat may result in less final thickness, and better pinhole filling.

Last 5 years or so I've taken to running a dry roller over it to even out the squeegee marks and leave a stippled surface, which becomes a nice sanding guide.

I also brush on a low viscosity epoxy, then simply roll it.



West 105 with 206 is medium to low viscosity (725cp) among laminating resins, but jumps up to 975cp with 205 hardener. System Three Clear Coat is about 400cp. MGS 285 is 300-500cp.

Main thing is to find a method that works for you...epoxy choice being only one part of the method.

Many years ago I used to do a lot of Corvette repair and restoration and we used a product called Featherfill to block the waves out of the fiberglass. It also worked very well for the everpresent pinholes.
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Featherfill has long since fallen from favor among glass airplane builders. Personally I refuse to put any polyester product over epoxy, having had an early project blister and peel after a few years.

I'd suggest sealing/filling, then epoxy primer followed by a good high-build urethane primer/surfacer for blocking. PPG K36 or K38 come to mind.

Off to the shop now...epoxy sealing some mold plugs I sanded out this AM.
 
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DanH said:
Dave, that might be more application method than epoxy choice, and hardener choice can make a big difference too.

If you just paint West on, it will be pretty thick. Squeegee off all the excess, allow it to jell, squeegee on another coat, gell, repeat may result in less final thickness, and better pinhole filling.

Last 5 years or so I've taken to running a dry roller over it to even out the squeegee marks and leave a stippled surface, which becomes a nice sanding guide.

I also brush on a low viscosity epoxy, then simply roll it.



West 105 with 206 is medium to low viscosity (725cp) among laminating resins, but jumps up to 975cp with 205 hardener. System Three Clear Coat is about 400cp. MGS 285 is 300-500cp.

Main thing is to find a method that works for you...epoxy choice being only one part of the method.



Featherfill has long since fallen from favor among glass airplane builders. Personally I refuse to put any polyester product over epoxy, having had an early project blister and peel after a few years.

I'd suggest sealing/filling, then epoxy primer followed by a good high-build urethane primer/surfacer for blocking. PPG K36 or K38 come to mind.

Off to the shop now...epoxy sealing some mold plugs I sanded out this AM.
Click to expand...

I guess to each his own. I still use feather fill on projects including aircraft fiberglass parts. I have yet to have a blister or pinhole open up. I must be lucky...
 
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