Estes Mini Arcas, #2441, Part 8, Fin Primer & Sand

A strip of tape was set down the root edge to prevent the primer/filler from soaking in.

The tape strip was needed because the root edges weren't  straight. 

The fins were set on scrap cardboard for the filler/primer spray.

Here's the before and after sanding the fins and short launch lugs.

The filler/primer soaks in a bit and helps seal the surfaces.

For me - Balsa grain filling and tube spirals filling is a two-step process - 

STEP 1: After applying and sanding the Carpenter's Wood Filler (CWF) 

STEP 2: The sprayed and sanded filler/primer fills in any remaining grain.

Everybody has their best methods.