Artist Carrie Lewis recently wrote an article on Empty Easel- What Should I Do with All My Old Art?.
It has some very helpful information about the proper way to paint on top of an old oil painting to insure that the paint will adhere and cover the old painting. This is if you want to leave the old painting as it is and paint over it completely or partially.
I assume that if your painting has a final varnish you should first remove the varnish with solvent before you proceed to the next step.
“To paint over an old oil painting, wipe it first with half an onion to loosen and soften the paint. Then wipe it with either linseed or walnut oil (depending on whether your paint is ground with linseed or walnut oil). Spread the oil around with your bare hand, then use a clean paper towel to remove the excess.
If you want an opaque surface to paint on (so the old painting doesn’t show through) paint a layer of burnt umber over the prepped surface. Burnt umber doesn’t fade and it remains opaque. Give it two or three weeks to dry and check the paint film by scraping it lightly with a razor blade. If you get a light powder, you can safely paint over the burnt umber. If you get a thin roll on the blade, let it dry another week. This step is important because if the burnt umber isn’t sticking to the old painting, the new painting won’t stick either.”
Read the rest of Carrie Lewis’ article on the Empty Easel website: What Should I Do with All My Old Art? where she mentions other ideas including scraping the old paint off before you begin a new painting.
This excerpt is used with kind permission of the Empty Easel website.
