I have a couple of roughly fist-sized plastic† pieces that I'd like to paint (I'll post a pic as soon as I can); preferably with a spray but brush is OK too.
First I intend to lay down a primer (for this material any ABS / styrene primer will do, and I know at least Montana and Tamiya brand plastic primers definitely work). Then I'm going to paint. Solid color, no fine details.
The thing is, this plastic is mildly flexible. It's not straight up bendy or squishy, but it's more like a really stiff rubber, like the kind of stuff you might see used as feet for cheap metal furniture (that's the only example I can think of). It's got a "softness" to it like... you feel like you will be able to bend it but you kind of can't, if that makes sense. So it's not going to be all over the place but it is not perfectly rigid.
Because of that, I'm worried that an enamel paint (it's all I'm really familiar with besides "latex", as I usually work with metal and wood) will start to crack and chip fairly quickly. I've also got my fingers crossed that the primer won't do the same thing, since I have zero experience with it.
Anyways, that was a super long-winded way of saying: What kind of paint can I use? It's got to have some pliability / elasticity when dry, and should at least be compatible with whatever ABS / styrene primers are made out of. Also, the pieces won't be handled super roughly but they will definitely get handled frequently, so it's got to be at least somewhat durable.
Bonus points if it blocks UV, but that's really just icing on the cake (also the primer might, not sure, will have to look it up).
† It's actually resin, not plastic. Specifically it's FormLabs Durable resin off an SLA printer, and the pieces are thick enough to make it rather stiff. But ABS primers are manufacturer recommended so it's simpler just to say plastic here (I mean, it's a polymer at least). It's mechanically closer to a polyethylene or polypropylene plastic, and chemically closer to ABS, than it is to a polyester or polyurethane resin.