Originally Posted by King Mouse
Surface prep is one major reason new paint will peel off of existing paint. The old clear must be scuffed to give the new base a 'tooth' to grab onto. If done right, it will be fine.
^^^^ What he said. If it was peeling, it was most likely not prepped correctly. Painting over existing paint is fine as long as you don't exceed a specified paint thickness (mills) and the surface is correctly prepped.