Estes Paveway III Build, Part 6 Fin Grain Fill - WARPING?

Here's the rear of the kit face card showing how the fins are built up.

There's no way to get a good grain fill after everything is glued together. Best to fill the balsa now.

The reinforcement plates were die-cut from 1/16" balsa. The balsa grain runs down the short leading edge, not down the root edges facing you in the picture.

I've read about it on the forums before. 

Warps are caused when filling thinner balsa with water based CWF.

Brush it on one side and the balsa will warp as it dries. The balsa will curve up on the coated, wet side.

Let the one side dry a bit, no need to wait until it's totally dry to brush the filler on the other side.

Here's all eight of the reinforcement plates - after both sides got a coat of CWF and had dried.

All eight pieces are dry and flat!

I didn't have to let them dry under waxed paper and under a weight or in a heavy book. 

One side of the drying CWF filler counteracted the warp from the first coat.

While doing a dry fit of the fins I noticed there was no recess cut in the root edge for the Mylar retaining ring inside.
(The Mylar ring was marked with a permanent marker so you could see it in the picture.)
One fin was marked for a notch. The other fins were stacked on this fin and all were marked.
The notches were cut with a sharp knife.