I am having a bit of a nightmare time with applying vinyl to foam PVC sheet.
I either cut it with a Stanley knife or a jigsaw then sand or file smooth. No matter what I do I seem to get fibres and plastic grain trapped under the vinyl making the vinyl look terrible.
I have cleaned the bench down.
swept up around me.
cleaned the sign repeatedly, but still, I get crap under the vinyl.
The best thing I have found for years is Body Shop Tack Cloths, the things Car Sprayers use before painting. Available from Halfords. Wipe seconds before you fit the vinyl. Only cut foam board with a knife, never sand it after, if you must, then do it after fitting the letters. Or as Alex says fit before you cut the board if you can.