Re: Exterior Plastic Paint
If the box is METAL acrylic should stick well so long as the box was thoroughly cleaned and oil-or-soap film free. If the box is plastic it is either PVC or polyethylene. If it's PVC (and is thoroughly cleaned yada yada), acrylic paint will stick to it. If it's PE, there are special primers (I believe a company like ZINSSER) for untreated PE.
Now to another issue - polyurethane (PUR). PURs adhere very well to acrylics. BUT many types of PURs are not UV-stable and therefore not designed for exterior use. Traditional methods of failure include cracking and peeling. If the PUR has a better bond with the acrylic than the acrylic has with the mail box... You fill in the blank.
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