Grout

[kinky]

Is this good for a fake rock? would i still need to seal it even though it says its waterproof??

http://www.focusdiy.co.uk/Decorating-Accessories/Unibond-Sandstone-Floor-Tile-Quick-grout-5kg/invt/185070

 

[garnothegreat]

if it has the sand added it will be harder to get the right viscosity to cover the foam and will flake off easier

 

[helix321]

I've think this should work fine you may want to add some acrylic modifier to help with consistensy and working time - Personally I've used sanded grout on many projects and have not expeienced any issues with flaking or other failure - too much water in any cement based product will cause premature failure.

 

[Seven65]

The reason you need to seal it is because it is corrosive. You can just use a basic tile sealer. If you're going to use grout you're probably best off making yourself a structure out of styrofoam first and then just covering it in a few coats of grout.