this post is a kind of tutorial of how I coat the foam rocks-
I use a combination of fine black sand, powdered clay and powdered soil
soil and sand comes out like grey river stone, clay and sand comes out more like sandstone or mudstone.
this time round I ground the dry clay and soil with a pestle and mortar
a close up to show how fine the sand is + a pic of the dry mix.
first I paint the foam because the rough foam has a huge surface area that soaks up loads of glue/epoxy.
this time i did the first coat with exterior pva glue instead of epoxy. Its much cheaper and easier to apply than epoxy. I wouldn't do this to rocks that were to be submerged though..
I painted a thin coat of epoxy and then dusted it with the sand/soil mix
at this stage it should look like dry dust. more sand/soil needs to be applied as it hardens as epoxy soaks to the surface and make wet patches-
If you don't re coat it as it dries it will end up looking dark, glossy and not stone-like
use a heater to harden the epoxy faster. I rubbed in the sand/soil mix into the shiny spots as they got less tacky
this is how it looked fully hardened.
here are close ups of the washed and dried surface-
I think it came out better than my earlier attempt because I ground the soil and clay into a finer powder.