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Odd.. could it be the PLA you are using maybe? Maybe some odd additives in it?
Or are you doing anything to the material after printing, like acetone smoothing?
I print with Hatchbox PLA, and have had no issues painting whatsoever. I don’t wash or anything, that’s usually only needed for molded plastics, to get rid of the releasing agent.
I prime with the Rustoleum 2X paint and primer spray for priming. just a single base coat. I find it to work best if you let it dry overnight to get rid of any tackiness.
For the actual painting, I use Army Painter for large areas, and Citadel Color for details, metallics, and shades.
But haven’t had any problems with the paint sticking at all… applies fine and drys pretty quick.
Maybe try a different PLA??