Just wondering about the foam you're using. There are some real high quality foams available used for prototyping - that give a very fine finish. REN-Board is something that comes to mind - try a google on it - you can get (used to anyway) a sample pack to try the various densities and even a small hunk of machine (blue) wax.
Might check the speed and feed or the cutting tool you are using. Hope thats all helpful.
:cheers: Jim
Experience is the BEST Teacher. Is that why it usually arrives in a shower of sparks, flash of light, loud bang, a cloud of smoke, AND -- a BILL to pay? You usually get it -- just after you need it.