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Old 05-06-2020, 09:57 AM
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Doug Sams Doug Sams is offline
Old Far...er...Rocketeer
 
Join Date: Mar 2005
Location: Plano, TX resident since 1998.
Posts: 3,965
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No doubt. My technique evolved.

Originally, I brushed on thinned Elmer's Fill'n'Finish directly on the balsa. But I found that didn't always fill all the grain. So I added a new first step of brushing on a primer of white pigmented shellac. It dries fast, and helps seal the wood. Plus, because of that, it's less prone to warping the wood versus the watered down FnF going straight on the balsa. Then apply the brushed-on, thinned FnF, and let it dry overnight.

Next I sand with 220 grit. If I then find any grain showing, I use an automotive product called body putty, I think, that comes in a squeeze tube. It uses a fast drying hot solvent, and is applied with a gloved finger. It goes on in a thin layer. It can be sanded in a short time.

Next, one or two coats of spray-on automotive primer (rattle can). If you don't get it on too heavy, it can be sanded 20-30 minutes after the last coat, then ready for a finish coat.

The reality off all this is that there is no sure-fire way to always get it done in the same number of steps. With each loop of applying a coat (and maybe sanding), you have to inspect for the grain showing, and be willing to re-do that step as necessary.

And, above all else, before spraying anything - primer or finish - use a frickin' tack rag to get the dust off. I've seen too many times where dust and dirt were showing thru the finish coat on somebody's rocket


Alternative: A couple coats of brushed-on hot solvent based model dope followed by sanding, then a couple coats of primer.

Either way, the key is getting the balsa stabilized/sealed with the first coat, so that the succeeding coats build up rather than soak in


Don't Like: Wood glue or thinned wood glue: This is a great adhesive, but it is way too difficult too sand. I avoid letting it build up anywhere. (While I apply glues fillets to fins, I don't let them get too fat, and always apply FnF over them to get the desired, smooth fillet.)

Doug

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