This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From: JCook@Epoch.COM (Jim Cook)
Skip using glue W/ balsa dust, dope, or any other junk for filling the
grain in balsa fins or nose cones. Use Elmer's "Fill 'n Finish" diluted
with water to a thick paint (like white glue is) and paint it on.
Non-toxic and a coat or two will do. Use Elmer's "Carpenter's Wood
Filler" thinned similarly to fill the spiral in body tube. Both come
in a white plastic tub with an orange lid. Note - the latter is
harder to sand, so don't make the mistake of using it on balsa as it
will require a lot of sanding.
From: firstname.lastname@example.org (C. D. Tavares)
Fill your fins BEFORE attaching them. (Don't fill the root edge).
From: email@example.com (David M.V. Utidjian)
[To hold fins in place and aligned while drying I bought an Estes
fin alignment kit]. At 15-16 bucks it seems a bit
expensive but is well worth the aggravation and time it saves. You
can even make your own if you are handy. I just set up my body tube
in the jig and then check the alignment of all of the fins to the body
tube. Then I use a thin bead of 5-min. epoxy. and in ten minutes I am
done. When I do the fillets I can do them all at once but don't have
to worry about the softening the glued on fins so they droop. You
still have to lay the model on its side though but only for 5 minutes.
From: firstname.lastname@example.org (Bob Kaplow)
The Estes Fin Alignment Kit has now been discontinued for '96. You can
use the newer Rocket Builder's Marking Guide ($5-8) to do one fin at
a time. [Editor's note: This is a HIGHLY RECOMMENDED item to purchase.
It provides everything needed to mark BT-5 through BT-60 body tubes
for 2, 3, 4, 6, or 8 fins.]