Here's what works for me...

First thing when I get a kit, I inspect the balsa nose cone/transitions for smoothness... some are really nice, some tend to be pretty rough, like the balsa 'tore out' when it was being shaped, and leave a 'scalloped' surface behind. If necessary, sand with 220 grit to get it pretty smooth and even, but I'd say 80% of the time this step isn't necessary.

Then, using some ultra-thin CA (I prefer the cheap stuff in the pink-labeled bottle from Hobby Lobby, which you can get with the 40% coupon on the weekly special) wet the surface of the nosecone down with the CA (do this outdoors and stand upwind or crosswind-- the fumes can be rather eye-watering!) and I usually start at the shoulder end of the nosecone (only apply to the part of the cone OUTSIDE the rocket, not to the nosecone shoulder itself, as it will *slightly* increase the diameter and require sanding down to fit the tube well.) Apply a thin, even layer all the way around the cone, usually twice-- the balsa will "suck" the stuff right into the wood fibers. This greatly strengthens the wood and seals the pores as well. Carefully set the finished nosecone aside to 'dry' for about 10 minutes or so, on a cardboard box or wax paper (so if any glue runs down, it won't glue itself to what it's sitting on).

This tends to raise the dust and loose fibers on the surface of the nose cone, so a light sanding with 220 grit will get rid of all this stuff and make the nose cone very smooth again fairly quickly.

Take your Elmer's carpenter's wood filler, and using a small baby food jar or suitable container, mix about a teaspoon of it with several drops of water, and work it up until it's smooth and about the consistency of hot dog mustard. You want it thin enough to BRUSH on, but not so thin that it's watery and makes bubbles... just about like bottled mustard. Using a 1" cheapy paintbrush, apply the thinned FNF to the nosecone/transition and set it aside to dry.

Sand with 220 grit, taking MOST of the filler back off. Reapply if necessary, let dry, sand it down with 220 again. By this time the cone should be about 95% filled and smooth. Sand again with 400 grit to make it slick... you just want to get it nice and smooth and ready for primer-- if you can see any defects at this point, now's the time to fill them with another spot application if necessary.

Give the cone/transition a couple coats of good filler/primer (I like Walmart Colorplace Hi-Dep Red Oxide Primer, but they quit making it, so now you can only get the regular gray primer, but it works well too, and of course Dupli-Color and other "high solids" primers will give you and excellent job as well, for more $$$. The main thing is, make sure the primer is compatible with whatever paint you plan to spray over it.

Once the primer is dry, sand with 220 grit, until it's as smooth as you can get it. If you sand through the primer anywhere trying to get everything smooth, reapply, dry, and resand. Once it's smooth, switch to "damp sanding" with 600 grit wet/dry sandpaper dipped in a bowl of water and shaken off, use just enough moisture on the sandpaper to prevent it from clogging. Sand the whole surface in small circles, moving constantly and overlapping the previous passes. Periodically wipe off the 'sanding mud' (primer dust mixed with water) with a damp paper towel. Hold the cone up to the light and sight down it like a gun barrel, and slowly turn it... the 'glint' of light off the surface should be smooth, even, and unbroken, not wavy or showing any other visible defects. This whole primer sanding and resanding may take 1/2 an hour to an hour to do per part, but it's worth it.

Wipe the finished part down with a damp paper towel to remove all traces of dust, and wipe down with a dry paper towel and set it aside to dry overnight. They'll be ready for paint in the morning.

Once spray painted, the part should look identical to a plastic part. I've been amazed at how smooth balsa can turn out with a little time and elbow grease using these simple methods...

For fins, the same methods can work, but it's MUCH simpler to simple glue a layer of printer paper to the fins to cover the wood grain... and it strengthens the fins SUBSTANTIALLY where simple FNF does not. I cut and stack-sand my fins to identical shape, sand in any desired airfoiling, then get a few sheets of printer paper. Cut a sheet of printer paper into pieces about 3 times the size of the fin... enough to cover both sides of the fin at once and still have at least an inch of paper all the way around the edges of the fin. In fact I usually lay the fin on the paper, about an inch in from the edge, with the LEADING EDGE of the fin toward the center of the paper, trace it lightly onto the paper, and CAREFULLY ROLL THE FIN OVER THE LEADING EDGE, and trace it again to give a "mirror image" of the fin. Apply a thin, even coat of WHITE GLUE (VERY thin, but even!) to the entire surface of the paper where the first side of the fin will be glued down, extending past the outline of the fin, and gently press the fin down inside the first outline. Apply a second thin layer of white glue to the exposed surface of the fin (yellow glue isn't really any stronger here and white glue is MORE than adequate to the job, shrinks less, and smooths out easier, and is preferable on paper/wood joints instead of yellow glue). Once you have a THIN even coat on the entire exposed surface of the fin, gently roll it over THE LEADING EDGE onto the other half of the paper. Use a Sharpie marker or other 'fat' round pen or object (Sharpie markers are darn near perfect to the task, though) and gently "burnish" the paper down tight to the fin, starting from the center of the leading edge and working toward the root and tip edges, and back toward the trailing edge-- rolling/pressing the paper down tightly against the wood, and working any excess glue toward the edges of the fin. Flip the fin and repeat the burnishing, finally burnishing the paper down to itself around the edges of the fins, and set them aside to dry (usually overnight-- I do this the same day I do the nosecone filling/priming/sanding).

When dry, cut the fins out of the paper with scissors-- cut down to within about 1/4 inch of the edge of the fin. Using a SHARP hobby knife, gently 'shave' the remaining paper off the edges of the fin down to the balsa on the root, tip, and trailing edges (the paper should be tightly glued to the leading edge and smooth) and presto-- finished fins ready to glue on the rocket, suitable for priming/light sanding/painting. Having the paper wrapped and glued over the leading edge eliminates the possibility of delamination from the airstream ripping past the fins in powered flight, and the paper EASILY doubles the strength of the fin (or more) while adding only VERY MINIMAL weight... works like a champ!

When done right, the whole rocket will look like it's made of plastic... Also, tube spirals and any other imperfections that need a bit more than primer to fill, I find that the autobody 'spot putty' sold in tubes at auto supply stores works MARVELOUSLY on... just dab a bit onto the spot, and sand it down smooth when it's dry. The stuff is basically primer solids with just a bit of lacquer thinner in it to make it a paste...

Good luck! OL JR

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