Never underestimate the utility of a decent primer. I've probably ruined a Cobra Rage tank, but always buy the good stuff.

I bought a can of the Wal-Mart brand gray primer. Crap flakes right off the old Joe plastic. Next time, it's Model Masters for me.

pfunk

Surface preparation means a lot, too. I had Model Master acrylic primer come off a model once, because I didn't have the plastic as clean as I thought I did.

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I used the WalMart primer on a Sharc 9000 and didn't have any problems with it chipping. Could be you might have sprayed it on TOO thick maybe? I then went over it with the Krylon Fusion Camo green and Black and have had no problems with chipping.

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The best Primer I've ever come across is http://www.thewarstore.com/armoryspraypaints.html armory primer. It is meant for tabletop gaming models and miniatures, so it really covers a wide variety of surfaces effectively.