Pretty sure you have to use a primer on them first, because the plastic is absolutely not conducive to painting. Every time I've tried, I've been frustrated with the results, and the only one I was ever even moderately successful on was a Megatron from the first movie, and I basically painted him totally black and drybrushed the metallic colors on there. You need at least a coat of primer to get the paint to stick.
As far as transforming a repainted one goes...that's damn near impossible without something being chipped off.
Theres a few things you need to do actually, not
just use primer.
For starters you need to give the toy a thorough inspection and find all the possible rub points, then you need to sand them down just a little to allow for the extra mass that the primer and paint adds.
Then you need to clean all the parts by soaking them in warm soapy water for a few hours.
This cleans off all the mould release residue and oils that can get on the plastic and stop the paint from adhering. You can use the strongest primer in the world, it still wont stick if the surface is dirty and oily.
Once all the parts are clean and dry only then can you start to prime and paint.
If all done correctly you should have a paint job thats durable and wont chip.
Chipping can happen on the best of paint jobs, dont beleive any customizer that says his paint is chip-resistant as its simply aint true.
Any paint will chip off given the right amount of force, but the trick is how to minimize it and the best way to do that is to follow steps like those above.
Using the right paint helps too. Acrylics tend to be weaker than enamels, but i prefer to use automotive paint. Strongest paint there is.
