I see that no one answered your previous question, so I'll try to answer both.
I don't know why you want to use nail polish on your model, but it is basically a lacquer and you might have to thin it with lacquer thinner. A very soft flat brush should work.
How to remove the old paint depends on what sort of paint it is. You will have to experiment. I have used liquid Drano on plastic kits. Obviously, gloves and eye protection are a good idea.
If you really care about a particular model, you may want to get one you don't care about to try things out on first.
Don