I've used the light weight spackle quite a few times as a "leveler" on balsa. I thinned it with water to make a light slurry and painted it on. I've never had a warping issue. It dries fairly fast, like a WBPU does. I guess my only concern, using a wood sealer, is not having the first coat of product soak into the wood providing the best bond possible with the glass cloth.

I guess it comes down to what ever works for you.

carry on

Ken