There is another alternative, and since you are using the floral foam (hopefully the hard cell variety and not the microcell that often disintegrates), after carving, use a hair dryer on high, or get an embosser.
When I carve foam, and want to seal the surface to create a good paintable texture, I use an embosser heat gun lightly over the surface. It gently melts the surface to a flat stone-like appearance. This will take first, second, and drybrush coats excellently.
Or if you prefer the rough exterior, just paint over what you carve/cut:
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This was a decade ago, but if I do it over, it would be with the embosser to ceate that flat stone look.
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