I don't know too much about electroplating, but I do know quite a bit about electricity. However, the name itself suggests you need good conductivity to plate the item via electricity. So, no plastic or paint that would inhibit the operation. And if you sanded down the paint, you'd probably want to polish the metal, so the electroplating comes out smooth and polished as well.
Like I say, I don't know too much about this, so I could be wrong, but that is my guess.