My normal sealing process for average thickness balsa fins is to apply one or two coats of watered-down Elmers Fill N Finish and sand smooth between coats. I do this before attaching the fins to the airframe. This fills most of the grain and also normally corrects any imperfections created when cutting the fins, etc.
The next step is for strength and to fill whatever grain is left. I apply Bob Smith brand InstaCure Thin CA (blue bottle) to the surface of the balsa. I do this by just applying it to the surface and then immediately wiping the excess with a paper towel. This smooths it out and creates a thin layer that dries very quickly. I try to work my way from one side of the fin to the other and will wipe the CA towards the balsa that hasn't been covered yet. Don't CA the root edge of the fin or you might have problems getting it to stick to the airframe later!
The CA will penetrate into the balsa and strengthen it considerably. You should be able to touch the fin without getting stuck to it within less than 5 minutes. A key here is to sand the fin smooth within 15 to 30 minutes. The CA is much easier to sand sooner since i believe it takes it a few hours (i've heard up to 24 hours) to dry fully.
Something else that i've been doing on recent builds is to leave a line of balsa along the root side of the fin CA free. What i mean is the sides of the fin closest to the root edge, not the root edge itself. This is to allow the glue you use for a fillet to penetrate the balsa as well since it may be a different glue than CA. The amount of fin you leave should be determined by the size of the fillet you intend to produce later in the build.
As for primer, i have tried quite a few (though i still need to try Kilz) but i am stuck with Plasti-Kote Spot Filler and Primer (see here:
https://www.rocketreviews.com/tips_emrr.shtml#kraft_paper_tubes). The stuff is awesome! It can be dry and wet sanded. When sanding it dry, it turns into powder instead of the gummy stuff that sticks to your paper. You can actually get this primer to shine like paint when you wet sand it. I've done some side by side comparisons with krylons and duplicolors and this stuff wins in all categories. I still need to try Kilz though
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One side note here:
I recently built a competition model that had super thin balsa fins (1/32" if i recall). I tested Fill N Finish on some of the scrap balsa from the kit and it warped pretty badly. I decided to just use CA by itself and it did the trick perfectly. I think if i hadn't used CA on the fins they would have broken on the first flight easy.
More Fun:
I have experimented some with how much water to add to Fill N Finish and have found some neat things.The more water you add, the easier it is to sand later (i know it sounds obvious but it didn't occur to me until a while after i started using it). I found that applying it somewhat thick like maybe chocolate syrup is good for the fins as i described above. For thin fins, you may want to go thicker or even straight from the can to prevent warping the balsa.
Another nifty thing i found is using it for smoothing fillets. The first time i tried using Fill N Finish for fillets i didn't use much water and the fillets were very difficult to sand into shape. I tried again though after hearing someone mention thinning it more and it works great. This time around i used almost a water consistancy and it worked great! There are a couple of drawbacks though. It takes longer to dry and requires up to three or four applications (depending on how big you want the fillet). The other drawback is that it's weak (but that was sort of the purpose too
). As long as you applied a good fillet of glue under the Fill N Finish, everything should be good. The next step would be to sand the fillet smooth. I generally take an 1/8" dowel and wrap it with 400 grit. This will easily shape the fill n finish fillet. The only other step i would recommend is to put a drop of thin CA at one end of fin fillet and let it run all the way across the fillet. This will strengthen the fillet, as well as the thinned fill n finish fillet.