Sign shops use a laquer based paint for the plastic sign panels. If you just want to spray the background color, get a rattle can of the color [in laquer] you want and spray it. [also a coat of white laquer over the color will distribute the light] Flip the glass over and apply the decal. In your case put a light behind your glass and spray light coats and watch how the light is shining thru.

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ROD [Mich.]

ROD [Mich.]