I see an increased rise in "calibran green" here XD. Joking aside

I have a few questions again:

Do you paint over the old colour with new paint or do you clean it first? If so, with what ?

I've been looking through the internet and I've seen some figures that fit my Dark Angel army very well. I also want to have one or two sub-orders of the Dark Angels.

If you paint them over, isn't the problem that they look too "fat"?

I look forward to your reply