Just so we're clear:I'm referring to Dow's Styrospan,in 1 inch panels.
Like others here,I've despaired of finding a clean way of cutting these panels.This afternoon,I spent time with my styro,shaping the form for a fountain.I have Exacto knives,bread knives,hot cutters and my power tools.Well,I found the most accurate,and least hassle,was using my variable speed jigsaw.Using a standard woodcutting blade,at a low setting,I was able to efficiently cut,with good accuracy,quie a few pieces,with little detritus.At higher speeds,it seems to throw off a lot of junk.Lower,it meant that I ended up with a coarse powder,easily cleaned up.Wore my mask to prevent inhaling any fun bits.I'll still use my hotknife(sucks batteries!),but the accuracy is not as good.
Maybe for my future septic covers...?
Just thought I'd pass it along.
Cheers from here