I learned this trick from GMobile17 and it has made my life a lot easier!
An easy way to strip off acrylic paint from your resin kits is to use Oxy Clean/Vanish.
Here I'm actually using a cheap brand from a local discount supermarket and it works just as well, so I guess this stuff is all the same.
To show you how to do it I'll use a 1/8 Cutey Honey based on a design by Sorayama-san. It's a beautiful sculpt, but it's a Thai recast from Hobbywarrior: quite poor and brittle resin. I messed up with it years ago and it's time to give it a good scrub and start over.
So, I add a little less than a scoop of this stuff
in the container where Cutey Honey waits
and I add boiling water from the kettle. If there's the risk of bending resin parts then use hot and not boiling water. However, I find that boiling water works better. Be also careful to use a suitable container for this.
Leave overnight or even 24 hours. If you can, it would also help to freshen the Oxy soup at one point.
This is what your resin should look like with the aid of a toothbrush and some scrubbing.
Ok, I realize now that this photo is way creepy as it looks like someone chopped up Cutey Honey pretty good!
Anyway, in my case it took some more scrubbing and a little scraping with a knife to get to this point because the chrome paint I used was particularly tough. Results are variable according to the kind of paint you used: I found that the brands I usually use, Americana and Createx, come off the resin completely after this treatment as does my Vallejo primer. This is also an excellent way to clean airbrushes from old paint (it does happen to leave the paint in there, doesn't it? I sometimes forget to clean them! ^_^") and to take off dry color from my mixing jars.