It has been almost a month since I last posted an update and not a whole lot has happened. The semester ended, I earned four A’s and one B and I’m not happy about the B, but it was what I earned. I am doing a lot of day job catch-up to try and finish a stack of projects before AMT school starts August 10th, and let me tell you, AMT school can’t start too soon. I am so excited to start this program. I have been catching up on all of my aviation mags, reading the federal regulations relating to AMT’s, and poking around in some of the other study materials as well as looking up info on my own airplane project.
Speaking about the airplane project… I have done almost nothing since February 6th and that makes me a little sad. Yesterday I was out at Kelly Moore picking up some materials for the day job and decided to ask the resident paint encyclopedia Dan about the properties of varnish and the thinning there of. Don confirmed some things I had heard before and gave me some new insight into the use of varnish for aircraft wooden structures.
Turns out that gloss finish is lighter weight that semi-gloss or matte finishes. The reason is simple. To reduce the gloss in the finish they add a de-glosser, small particles that make the surface a little less regular. The less glossy the finish, the more particles. This is an over simplification of the chemistry involved, but it is the basic gist.
So for airplanes we want the lightest coating possible to keep the over all weight of the airplane down. One thing done to reduce the weight further and improve the penetration of the varnish is to thin it with paint thinner. The thinner acts as a carrier agent allowing the varnish to go deeper into the wood structure and then evaporate off. It’s a win win, less varnish used and a deeper layer of protection. We varnish wooden aircraft structures to protect from moisture from water but it also protects from other things like oil, fuel, and anything else that would soak into the wood and cause or accelerate deterioration.
Just about all wooden parts are treated on all surfaces to protect them including in between parts where they are joined by hardware. This is one of those capture points that would accumulate moisture and trap it causing an acceleration in deterioration. Inside drilled holes is another place to ensure a good coating and penetration of varnish to protect from the corrosion of metal parts.
The parts that are not completely treated are the ones joined with adhesives. Many structural pieces are joined with epoxies. For the epoxy to penetrate and join the materials those surfaces must be completely free of varnish. The epoxy protects the surfaces it is in contact with.
After a joint has been made and cured, varnish can be used around the joint to protect wood not protected by the epoxy. But danger lurks here if you are not careful, if the varnish, or more precisely the material used to thin the varnish, reacts negatively with the epoxy it can weaken or even destroy the structural integrity of the joint resulting in catastrophic failure.
To mitigate the risk of this kind of failure, you need to do some destructive testing. Test the wood for penetration with different ratios of varnish and thinner, using different thinners, and those mixtures that work best on epoxied joints. Making sure to use the techniques and materials you would use on the airplane. Same wood, same epoxy, same varnish mix.
Ideally, the woods own internal structure will separate before the joint fails under static or dynamic loads. If the joint fails, its back to the bench for a new formulation.
All of that having been said, paint thinner is not the only substance that could be used to thin the varnish. Naphtha could also be used. Using Naphtha would decrease the drying time and generate more heat in the curing process, the unknown is how it will affect the T-88 epoxy I will be using. This means I need to do some bench testing. I will be laying out a short test program later.
That’s all for now, so if you have a chance, go punch some holes in the sky for me.
Blue skies, and tailwinds,