Spackle reduce resin penetration in blank. It reduce final weight but some kind of resin penetration in blank improve stability of foam between skin. That reduce drasticaly buckling of skin affected by compression…
As an airbrush artist I hate spackle. Spackle is a lightweight filler. Used for filling holes in your wall before painting. The main problem is that spackle has no binders. Binders are the bonding agents that make things stick to a surface. Just like the acrylic paint I use on the boards .I have even tryed to add acrylic to the spackle with varied results. Most shapers finish work is questionable at best. Give them EPS and that spackle saves the day! I paint boards every week that have been spackled then sanded back to bare foam. Since the spackle has no way of sticking to the foam, when I tape, it pulls the spackle right out of the foam. Leaving the holes just like the pre-spackled board. I then have to go back and repair the areas of the board were the spackle and the paint have come off. I have spent years trying to make this foam work and look like a Poly boards. In conclusion I have come to the opinion that for me, painting EPS is like airbrushing the lunar surface of the moon. Or the stucco wall on your house. And to address the resin and micro-baloons method it doesn’t work all that well for painting either. The paint needs to absorb to dry even. Paint also runs. And yes I have started with white base-coat. In conclusion, EPS lightweight,.yes More buoyant, maybe. Pretty as Poly, never. And yes, I do sell EPS boards. And no. I don’t spackle. For me, fine finish work is key for a light board.