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paint blisters on cowl

C

cjohngraham

Member
My RV-7A was painted 6 months ago by a very reputable shop. Lately the upper and lower cowl have erupted in blisters in multiple locations. The paint shop thinks an O-360 should not produce enough heat to create blisters, and they say this condition is very rare. Fortunately they will fix the problem--this time--under warranty, but I'd really like to know what's happening so it doesn't again.

Has anyone experienced this, and what was the cause? The paint shop (Master Aircraft Services) mentioned heat and fiberglass contamination, e.g. oil, as possibilities. The plane had been flying for 5 years before being painted and the cowls had spray-can grey primer on them. I'm assuming the shop took that primer off because they use a special $1200/gallon primer for fiberglass.

Another RVer asked if we had painted the inside of the cowl (we had not) because fiberglass can have pinholes that can let hot air through.

CHTs can run up to 420 (OK per Mike Busch) in climb, mid-upper 300s in cruise.

Anyone else had this problem?
 
Every cowl needs to have a good primer on the inside. I add white epoxy rattle can paint on top of the primer as a heat reflector. In addition I add FiberFrax under Van’s self adhesive aluminum heat shield, and heat shields on the one or two close to the cowl exhaust pipes. Depending on your exhaust you can easily scorch the inside of the cowl.

I would bet your blisters are due to contamination or poor surface prep.

Carl
 
Inside cowl

I skimmed with neat epoxy. Applied two coats of 2K White Epoxy primer. Lastly fiberfrax covered with Vans aluminum foil. Exhaust pipes have Anti Splat heat shields.
Have not flown yet.
 
No blisters 31yr 6A, nothing inside cowl, no paint, fiberfax, heat shields etc. I only have had/seen it due to contamination in surface before painting. Id sand out blisters and use epoxy primer over, sand repaint. Im no pro, this is what Id do.
 
No treatment of any kind on the inside of my James cowl, 1345 hours now over 7 years and no paint issues.
 
Sounds like a contamination issue. How well was the exterior of the cowl protected/sealed over the last 5 years? Even though the cowl was primed, were there any visible pinholes which might have soaked up oil? Was the airplane flown much/any prior to priming the cowl?
 
Just some neet epoxy on the inside of my upper cowl. No blisters after 15 years. Gotta believe the problem is with the surface preparation or contamination. The cowl and paint should be able to easily hold up to the heat with the possible exception of where it’s very close to exhaust pipes.
 
Cowling Prep

West Systems epoxy on the inside and I primed the outside. (Which the painter informed me that they never paint over a customer's primer but remove it just in case I didn't sand it enough, good call) No issues to date and holding up well. My FWF area can get to 175 F in the summer after shut-down. Anti-Splat exhaust shields used in multiple locations. Nice shields.
 
Some cowls are porous and will get oil inside them. Sometimes I’ve gotten water in them just from washing. Both could cause blisters. Also could be a contaminant on the outside. On my -8A I got a couple of heat blisters from the exhaust. Once I fixed it and put in a heat shield I didn’t have any other problems.
 
Paint blisters are usually due to contamination before paint or poor preparation. Yes, the inside should not be left and should be sealed with something, a high temperature oil resistant paint would be best. Ask your local Diamond agent for some time expired internal cowling "paint" it is a high temperature resin with a pigment that has a short shelf life.
 
Sounds like solvent pop. It can happen when solvent gets trapped below the paint and cannot escape. Once it gets hot, it expands and creates the blister. Too common causes. Not letting the solvents out gas out of the F/G before using epoxy primer. This can take a LOT longer than most expect. Most recommend leaving it in the sun for a day if any solvents were used. The other is using non 2 part fillers under the primer. Believe it or not, there are still painters out there that use the red laquer filler because it is so easy to work with. That will all but guarantee solvent pop.

Some guys that aren't used to F/G will just use their standard practice for solvent evap on metal parts, which is pretty fast. F/G requires A LOT more time for evap.
 
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lr172 said:
Sounds like solvent pop. It can happen when solvent gets trapped below the paint and cannot escape. Once it gets hot, it expands and creates the blister. Too common causes. Not letting the solvents out gas out of the F/G before using epoxy primer. This can take a LOT longer than most expect. Most recommend leaving it in the sun for a day if any solvents were used. The other is using non 2 part fillers under the primer. Believe it or not, there are still painters out there that use the red laquer filler because it is so easy to work with. That will all but guarantee solvent pop.

Some guys that aren't used to F/G will just use their standard practice for solvent evap on metal parts, which is pretty fast. F/G requires A LOT more time for evap.
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You can use red lacquer filler if you know what you are doing. You can’t fill anything larger than a pin hole with it. Anything bigger and you will have a bad day. For most things though, I use glaze coat.
 
Glazing

Bandera said:
You can use red lacquer filler if you know what you are doing. You can’t fill anything larger than a pin hole with it. Anything bigger and you will have a bad day. For most things though, I use glaze coat.
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I've used it for 30+ years. No problems. Like you said. Super thin. Just filling minor imperfections.
20221028_160215.jpg
 
Mod

Bandera said:
Did you do that mod to the cowl? I like it. Looks sharp.
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Thanks.
Yes. It was done to allow the plenum to couple to the inlets with neoprene socks. Basically the inlets were bonded together then the top cowl cut away.
20221028_123019.jpg
 
Bandera said:
You can use red lacquer filler if you know what you are doing. You can’t fill anything larger than a pin hole with it. Anything bigger and you will have a bad day. For most things though, I use glaze coat.
Click to expand...

agree, but I was speaking generally and under the assumption that there are some lazy painters out there taking short cuts.. I sometimes use it for pin holes found in filler and only in areas that will not be front and center when viewed. Seems the very small amounts do not soak up enough solvent to be an issue. I didn't listen to the advice given on my first paint job and used a bit on a relatively small imperfection and paid the price for that.
 
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