? will styrafoam is insulation blue hold plaster

I use lightweight joint compound instead of plaster. I glue the Styrofoam together with Great Stuff.

You might want to fasten something onto the styro for the plaster to grab on - something like screening or chicken wire fence.  I think the plaster or wallboard compound could slip off over time.

I cast my rocks with and glue them on the foam with hot glue, and then fill in the gaps with Hydrocal. Seems to work pretty well.

If you  scar the surface with something like a wire brush it will hold very well.   Another way to etch the surface of any styrofoam  is acetone.  Photos of foam etched with 100% acetone.  You can dilute the acetone with water to make it less aggressive. I find that different foam densities need a different dilution so you must experiment.            j

 drywall compound "could slip off over time".   maybe in a 25 years or so.

Way back in the day, Hyrdocal was my go to product.  I'm talking 35 years ago plus.  Is is still made and who is the current manufacturer?  The foam should have some fairly good support though if you don't plan on putting any heavy loads on it, such as foot or something.

Why not just make your foam the rocks?  You just need three simple tools.

@Ron045 posted:

Why not just make your foam the rocks?  You just need three simple tools.

 

I like the look of carved foam and also use a light brushing of acetone / water  in addition to the knife and hot iron to sculpt it  However I don't like to use it in an area where kids can get to it. Many years ago after a divorce my layout ended up over in my brothers basement. One day the kids found that the mountains were soft and before anyone discovered their hijinks they had picked a big patch off.  No great loss since we had decided that we would pull  that layout apart and build an around the room layout but when we did we made the mountains from hydrocal and  drywall mud in areas where we wanted to carve because hydrocal is so hard to carve.  As to the original question hydrocal on roughed up foam holds fine and you could use it to glue small rocks to a vertical rock formation and hydrocal cures very fast so you don't have to hold the rock in place long.  I still like carved foam for rock faces as long as little kids can't reach it.                                   j

@GG1 4877 posted:

Way back in the day, Hyrdocal was my go to product.  I'm talking 35 years ago plus.  Is is still made and who is the current manufacturer?  The foam should have some fairly good support though if you don't plan on putting any heavy loads on it, such as foot or something.

Woodland Scenics carries Hydrocal Light - supposed to be just as strong and lighter.

Walmart also has it online.