Prepping ready-made canvas for oil painting
Factory made, ‘pre-gessoed’ canvas is great for acrylic paints but is not actually ready for oil paints. The following preparation will protect the canvas so that your oil painting will last and won’t crack or peel. I recommend treating the back of the cotton canvas with two coats of Golden GAC 400 medium, which is a fabric stiffener. This will tighten the canvas up like a drum to reduce the amount of flexing of the fabric under the dried oil paint. It also makes the cotton less absorbent to moisture in the air which is important as you don’t want the cotton to be able to absorb moisture once you have applied oil paint. The front of the canvas should be coated with two layers of Golden GAC 100 medium. This is key because oil paints are acidic so you need to seal the canvas to prevent the acids from ‘eating’ or rotting the fibers of the fabric.

Here is a step by step procedure for preparing ready-made, pre-gessoed canvas for oil painting:
1. Coat the back of the canvas with two coats of GAC 400 (I always use two to provide good coverage) allowing each coat to dry about 45 minutes or more in between.
2. Put two coats of GAC 100 over the gesso on the front of the canvas. I use a large, soft, 'house painting' type brush. Finish with brush strokes all going the same direction. When applying the second coat, finish with brush strokes going 90 degrees to the first coat. (this gives a bit of a canvas texture finish) Again, allow about 45 minutes to an hour to dry between each coat or until no longer cool to the touch or tacky.
3. Now you need two layers of acrylic gesso for the paint to adhere to. Finish each coat with perpendicular brush strokes as outlined in step two. I allow the first coat to dry for a few hours before applying a second coat. If it feels cool when you lay your hand on it, there is still moisture that needs to finish evaporating. After the second coat it is recommended to allow at least 24 hours drying time before beginning the painting.

Preparing Wood Panels for oil painting
Untempered Hardboard is one of the most archivally sound supports for painting, especially with oils. People often think of canvas as a historically traditional support but in fact wooden panels would have been commonly used before canvas. For small paintings, untempered hardboard is ideal. It comes in 1/8 inch or 1/4 inch thickness. For sizes larger than say about 18x20, the wood will be too heavy, so it is better to use canvas for larger paintings.
 
1. First sand the shiny side of the panel with about 100 grit sandpaper.
2. Next it is important to seal the hardboard as there are natural glues in the wood that could discolour the paints. I like to use GAC 100 mixed two parts with one part water. Or methyl hydrate mixed two parts with one part shellac also works well. Apply a thin layer of the sealant to both sides of the panel. Let dry completely.
3. Then apply a thin coat of gesso on the back of the panel. This helps to resist warping. Apply 3 to 4 coats of gesso on the front, drying between coats. I paint the gesso on using a large, soft, ‘house painting’ type brush. Finish with brush strokes all going the same direction. For each coat, finish with brush strokes going 90 degrees to the previous coat. (this gives a bit of a canvas texture finish) Allow each coat to dry for a few hours before applying the next one. If it feels cool when you lay your hand on it, there is still moisture that needs to finish evaporating. After the final coat it is recommended to allow at least 24 hours drying time before beginning the painting.