What do you think of the BABY POWDER and spray lacquer method of finishing HL balsa skinned wings?

From : Don Stackhouse

Baby powder is primarily talc. It's ground up rocks. We use it in gel coats for mold making sometimes. The first time you pick up a carton of the stuff you will change your mind about the wisdom of using it for filler! It's VERY heavy. The reason it seems lighter in the form of baby powder is because it's fluffed up and full of air, but when you add the lacquer you replace all that air with paint. Microballoons instead of baby powder would be lighter, but the times I've tried that technique the results haven't been all that impressive. If you're trying to achieve a glass smooth surface (which, as I mentioned in my post to RCSE, really doesn't accomplish anything aerodynamically on most HLG's), then fillers can be helpful, but the bottom line is that anytime you add more stuff to the surface you are adding more weight.

Do you believe that the VARATHANE is a lighter and/or better way to go?

In my experience, yes. Trying to add fillers to the Varathane doesn't seem to accomplish anything. The balsa is so absorbent that the Varathane (or other liquid material) soaks right in, leaving added solid filler materials (such as talc or microballoons) sitting behind on the surface. The wood below the surface gets just as saturated as it would have without the filler, probably moreso. This is because when you thicken the paint with fillers, you inevitably have to apply heavier coats to get it to spread (like a squeegie full, typically). Since there is a greater volume of fluid sitting on the surface with each coat, it can soak in farther before setting up. If you leave it thin, and put it on in EXTREMELY thin coats, there is so little on the surface that it can't soak as far into the wood before it becomes dehydrated and sets up. The depth of wood saturated in the finishing process is less, so the weight is less. The catch here is that you MUST put it on in VERY LIGHT coats, and you must let each coat dry thoroughly before applying the next. The first time you apply a coat too soon or too heavily, it re-dissolves the previous coats and they all start merrily soaking down deeper into the wood, which means you will need to add even more to seal the wood.

As long as you can keep the Varathane restricted to just the surface, it seems to be about the lightest finish available, with the possible exception of fogged on spray Aerogloss dope or lacquer, with no added fillers. The idea with them is the same, a very thin paint applied in such thin coats that they don't soak in. Since their highly volatile solvents evaporate faster than water or alcohol (the solvents in Varathane), the time available for these to soak in is even less than the Varathane, so it's possible to get them to stay in an even thinner layer on the surface. Once again, just one coat applied too soon or too heavily can destroy all the weight savings. The more volatile solvents also are much more prone to dissolving the previous coats of paint, so this method is even less forgiving than Varathane. One mistake and you'd be better off Monokoting it. This is definitely an area where "haste makes waste"!

Don Stackhouse @ DJ Aerotech