Foam Aquascaping?

[Cap'n]

Silicon is the way to go. You could even go so far as to build a frame (back, bottom, sides) out of pexi and do all your foamwork within. That way you can be sure of the dimensions, makes an easy job of glueing and you can competely seal off the foam from the water. Maybe even toss a couple rocks at the bottom of the foam stack before sculpting to give the silicon a hand.

I still think getting rid of all the foam after coating is the best solution but extruded foam is definitely an improvement over the expanding.