This is a question that has come up countless times over the years in my workshops and presentations. Let me start by saying, if you are concerned about permanence, esp if you are painting in oils, then don't do it. Get another painting support and start your new painting on a fresh surface.
The most common solution is to cover the old painting with gesso as a method for starting over or perhaps changing a certain area of the composition. This is not a good idea for oil, or acrylic paintings. Gesso is a ground, it is not paint. It is meant to be applied to an absorbent surface that has 'tooth'. Gesso does not have a lot of adhesive strength or flexibility like paint, so it will not adhere permanently to painted surfaces, esp oil paint, and esp on canvas.
If you decide you want to paint over a painted surface then here is what you should consider if you are concerned about permanence.
- Use white paint instead of gesso to cover the old painting or area you wish to work on.
- Remember that oil paint becomes more transparent with age, so the underpainting may show thru in areas if the new painted surface is not thick and opaque enough.
- If you do want to paint over an already dry oil painting, here is the best way to go about it to minimize later troubles:
1. Scrape off as much of the old paint strokes and impasto as you can with a palette knife. Use a flexible razor or blade to remove as much of the old paint as possible without scraping the ground itself. You may need to place a board under the canvas to keep it from sagging while you scrape.
2. Rough up the surface with sandpaper to give it ‘tooth’ so the next layer will adhere better.
3. Dust (vacuum) the surface and wipe it clean with a solvent like turpentine or mineral spirits.
4. Apply a thin coat of white or light gray paint.
- It is very important to remember that when you are sanding and cleaning the surface that pigment particles that are potentially highly toxic will be stirred and you must contain these and not breath them. I recommend using a vacuum cleaner and dust mask.