There is a practical reason for this rule – painting over usually doesn’t on a technical level. However, paint doesn’t erase – once it is on the paper, it stays. Even if I tried to paint over it, the dim form of the “mistake” would still be visible underneath.
There is deeper reason as well. One of the gifts of process painting is practice letting go of judgment and quieting our inner critic. The stronger the dislike, the greater the possibility that it is connect to deeper feelings about ourself, especially the parts of ourself that we don’t like very much.
How do we know something is a mistake? Our critic is telling us so. What happens if we ignore that bullying voice? This is a chance to find out what it is like to live with imperfection, accept it and (maybe) even embrace it. What if you understand this logic but you still really want to cover something in your painting? Then paint over it and see what happens. After all, process painting is about the experience, not the results. Accept it, paint over it, or transform it something else, as long as long as you are paying attention your choices, it doesn’t really matter which one you choose.