When I was terrible at painting miniatures, I was a huge proponent of the different prepainted miniatures lines that exist. I still think they are useful for those folks with no time to paint or without the talent to paint very well. However, since I've buckled down and learned a few things about painting minis, I now realize how truly awful most of the paint jobs are on the prepainted lines, and how a lot of the castings lack detail that would be present even in a soft plastic mini put out by a true miniatures company. The early D&D prepainted minis are especially awful. As usual, the Pathfinder line stepped up the quality, as Paizo does with every idea they take from WotC (but they are still not quite table-quality to me).
So, I began sifting the prepaints I had, looking for sculpts I liked, and after isolating them away from the total crap, I began looking for a way to strip the paint from them. I found out that it's impossible. The paint on these things, at least on the lines I researched, is baked on and coated with something, and is often part of the mini itself is some cases (the plastic is that color, too). Apparently the paint is vinyl-based, and might as well be solid vinyl when in its final form. So, no stripping possible, at least not without using an acetone-based chemical, which will ruin the plastic anyhow.
So, prime and repaint it is! I've selected about 100 different minis that I will be repainting, from various lines. I've found a few that I thought were good enough to not need a repaint, only a touch-up. But I like to start from scratch most times, so I got a cheap can of primer that I knew would stick to plastic, and have begun the process of making all those minis white, gleefully covering the bad paint jobs they formerly had. The good news: unless the paint is totally caked on (see early Mage Knight products), some nice detail will stand out when it's not covered up by the totally insane use of dark colors that seems to be ubiquitous to these lines. I've had a few really pop after priming, and was able to see so many things on the mini that I didn't see the first time (including massive flaws). In the end, I think I'll be able to get some serious mileage out of these, and as long as the paint isn't too think, I've just extended the minis I have available to paint!