I primed some resin castings with plastic primer and then applied enamel paint. Not a good idea, as the enamel paint won't take very well and has to be applied too thickly which looks awful. Perhaps I should have used metal primer instead. Or should I have used acrylic paint on the plastic primer? Can anybody advise on what's the best stuff to remove the enamel paint please?
Try brake fluid. Soak, testing every hour or so until the paint is soft. Then remove softened paint with a brush (a stiff tooth brush will do nicely). Use gloves, as brake fluid is unkind to your skin. You could also try Polystrippa or similar products sold for furniture refinishing. Any stripper may also attack the resin, so test on an unexposed surface first. You will have to work fast once the paint is loosened, and you'll need a second container with soap and water handy to dip the model into as soon as the stripper has done its work. Rinse thoroughly. Clean remnants of paint out of nooks and crannies with a pick, small screw driver, etc.
But there is no guarantee that you will be rewarded with a pristine resin casting after all that work. Sorry. (And don't ask how I know... :-))
Further (unsolicited) advice:
Do _not_ use plastic primer on resin. It's made for styrene plastics, not resins.
Resin castings must be thoroughly cleaned. The usual "wash in washing up liquid" won't remove the moulding release agent(s) used for resin. Use a citrus-based cleaner, then soap and water. When clean, you can paint directly onto the plastic with enamels and acrylics, but I prefer to spray the model lightly with light grey automotive primer as a neutral undercoat for the colours. The primer is dry and ready for painting when you can't smell it while sniffing the casting.
I recommend strongly that you avoid enamels, or else use a professional quality spray booth plus filters over your nose and mouth plus eye protection. The organic solvents used in these paints are nasty things. They don't do much immediate harm, but the harm accumulates.
If you find that "paint must go on thickly", something is wrong. You may be using the wrong grade of paint. Model paint is made especially to cover in very thin coats, unlike paint made for cars, for example. If you are using an airbrush, then the pressure may be incorrect and/or you may be holding the brush at an incorrect distance from the model. Also, it's better to spray or brush several thin coats until you've built a coat that covers instead of trying to cover in one coat. This is especially true of light colours such as yellow, which are more translucent than dark colours. Make sure you wait for th paint to dry between coats.
There used to be a paste called Modelstrip, I think it is still on sale. Comes in a tub and dries out but can be rejuvinated with water. I used this with success several times although as with the brake fluid the scrubbing can remove a surprising amount of detail - I had to replace all the handles and things on two passenger coaches - tedious!
Warning, some brake fluids will melt plastics (including resins) as soon as look at it, also if the vase material is porous the brake fluid can get absorbed meaning that nothing will stick (or if it does, stay long)....
The advice I get on a miniatures group with this question is the following, DO NOT USE brake fluid, but go to your shop and buy an engine degreaser, Castrol Super Clean seems to be the favourite and use that.