Not quite the same.. Nail polish remover I believe is Non Acetone. Honestly it should have done the trick. Safer to use thab Acetone actually.
Sanding isnt required unless you have rough spots after you remove the paint or are trying to paint over a scratch.. But I would personally sand any surface I would paint on. Look up a midsole paint guide on youtube for how tos. In case you need more help, that is how I learned it (in case you have trouble, I found a good one for you ). Theres alot of extra steps you can do to ensure the paint sticks better , however from the sound of things yout having issues either because a)not prepping the surface properly for the paint to stick on or b) not using the right type of paint.
What type of paint are you using... - Angelus? If so what type exactly and color? Dont use any paint that is "Flat" , i.e. Flat White or etc. I have heard Flat White is prone to chipping due to something in the paint making it brittle. I am thinking the issue has got to be the paint, it sounds like your prepping properly and you stated your even using finisher.
Ive read up some people who sware by adding finishers first then adding paint. Others who will seal the paint afterwards. Personally, if you have a pair of beaters, I would practice on those until you find a method that works.
Can you post pics . Is it chipping at one particular spot or is it randomized?