Paint blister proposal for Vans resolution

[bret]

Is there any data with using the green loc tite on rivets on QB tanks before paint?

 

[Paul K]

Add me to the list! After three years of fuel in the tanks and no blisters up to now, 5 showed up at the same time all on one tank. My tanks are factory built and I inspected them before I closed them up. Not a single rivet was visible on the inside, lights, mirrors, and a scope used for inspection.

Going to poke a very small hole in the blisters before they get big and hope for the best

 

[lr172]

I have the problem too, and researched this a long time ago. I slow built my tanks and all rivet shop heads are sealed inside. I have no fuel leaks at all (740+ hrs). I had the plane professionally painted by an aircraft painter. The paint blisters appeared only on the top of my tanks on the rivet heads and took several months after the painting was complete to manifest itself. After much research, I finally concluded it was a reaction of the paint thinner/solvents used in the primer/paint that reacted with the proseal causing trapped solvent gases to eventually cause the blistering. It took the heat of the summer as well to create this condition. Google "trapped solvent", "paint thinner blisters", "solvent popping" or some such. Apparently, some painters may use paints and thinners that aren't compatible or aren't compatible with proseal. Even the thinner in the paint, not the solvent used to clean before painting, can be the culprit. Bear in mind that many production planes don't have this issue because they use fuel bladders or internal tanks, not a wet wing sealed tank like we have. Here is a link to one of the resources, look for "Solvent Popping".

http://www.tat-co.com/tat-co/documents/paintdefectguide.pdf

I have some experience, having painted several cars as well as my 6A. Trapped solvents are a very real problem. I learned this lesson using non-activated filler once in a few small areas. Anything that can absorb solvents will cause blistering. The issue is not the prep, but the solvents in the paint itself. Most solvents evaporate out of the paint through the surface, but they only have a few minutes to release before the the paint cures and creates a seal. Anything below the cured paint layer will hang on to these solvents indefinately, unless they have some other way out, such as between the rivet and sheet. You won't notice the problem until you put the painted part in the hot sun. The heat causes the solvents to expand out of the material and cause the blister (remember the paint is flexible and it's bond is relatively weak). This is why you are only seeing the blistering on the top surface. You need the convective heat of the sun to cause enough heat to create the expansion.

I can't say this is happening in these cases discussed here, but I would experiment a bit with proseal to see if it is absorbing the solvents. The expansion, i.e. blister size, should be related to the amount of material that can absorb the solvents.

Larry

 

[rockwoodrv9]

Yeikes Paul - I hate to hear you are getting blisters. Your paint job is so nice!! God luck with the fix.

Add me to the list! After three years of fuel in the tanks and no blisters up to now, 5 showed up at the same time all on one tank. My tanks are factory built and I inspected them before I closed them up. Not a single rivet was visible on the inside, lights, mirrors, and a scope used for inspection.

Going to poke a very small hole in the blisters before they get big and hope for the best

 

[Paul K]

Larry,

If this is a solvent issue, why three years of fuel in the tanks to surface or even 5 years ago when I painted the wings? Dont understand.

 

[lr172]

Larry,

If this is a solvent issue, why three years of fuel in the tanks to surface or even 5 years ago when I painted the wings? Dont understand.

It would seem that there are multiple issues here. I did not intend to state that solvent pop is the only or even a common cause, only that it is a real phenomenon. Just trying to educate the community so that they can best troubleshoot. Solvent pop shows up as soon as you get meaningful sun exposure in warmer weather.

 

[scsmith]

green loctite

Is there any data with using the green loc tite on rivets on QB tanks before paint?

I put a drop of green loctite on each rivet of the tanks of my QB -8 wings, about a week before the wings were painted. I lightly wiped excess loctite off with a rag with acetone.

That was six years ago, in a wide variety of climate conditions. Sometimes outside in 100+F, mostly in a hangar in CA and OR, temps range from 20F to 105F.

I have one blister. Its on the bottom, along the rear tank baffle rivets.

I might have missed that rivet, or just didn't get enough loctite on it.