how do i paint engraved plexiglass without getting bubbles?

[Matt]

i have a cnc machine and am doing a job where the customer wants the letters engraved in plexi glass but in addition would like the letters to be colored. i have made samples with different paints and keep getting bubbles or air pockets. ive tried flame glazing the letters so theyare not that fogged white look, but this really only works on the edges not in the engraved letters. my idea is for a sand type of material that can be sifted into the engraved portions and melted in the oven.