Posted by:
jgragg
at Wed Feb 17 23:02:19 2010 [ Email Message ] [ Show All Posts by jgragg ]
Hi Mark,
I think Jeremy hit it pretty squarely. The green works fine (just from internet research) but it's quite expensive compared to the alternatives, which work great (from personal experience).
I like the extruded (pink or blue) stuff best. But expanded (white) stuff can always be had for free in apparently unlimited quantities. I've even used the peanuts layered with expanding foam, to make a bubbly-looking fake limestone. The only downsides to the white are 1) it is less dense (which after grouting & sealing doesn't seem to matter) and 2) its "sawdust" is more copious and static-charged (can be a little annoying).
I've only used knives to carve the foam, but there are lots of DIY videos on making hot-wire tools to cut foam.
As for details of crevices, etc - Jeremy's right, there no need to do that in the foam, they can be achieved nicely by sculpting the grout on rough-carved foam. But I think that's a higher-skill proposition, and also requires more grout (leading to more weight in the end). You can get great outcomes with more control and less time pressure by carving in the foam, you just need to take care to clean excess grout out your crevices with a sculpting tool after 5-10 minutes of setting-up. (That's the start of the same time window you'd be using if you were just sculpting the grout.)
A very useful tip - to get the first coat of grout stuck onto the foam, I strongly recommend first applying a layer of full-thickness concrete bonding agent (the same Elmer's-glue-looking, acrylic copolymer goop you can also mix with the grout for your coats, and which you can also use as a "plasticizing" topcoat). The adhesion of that first grout coat is improved immeasurably with this act. You can apply the grout as soon as you have your "Elmer's" coat on the foam.
You can also use the "Elmer's" between grout coats. This allows thinner-mil grout coats, which aid in maintaining the details you achieved in the foam carving, and reduces the sculpting required in the curing grout.
Cheers,
Jimi
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RE: Best foam for faux rocks? - jgragg, Wed Feb 17 23:02:19 2010