How to prepare an old painted canvas.?

I have two very large canvases that I would like to paint on. Money is short and these size canvases are expensive. They are painted with oil and I work with acrylics. How do I prime them so that the paint doesn't crack later?How to prepare an old painted canvas.?
first sand the old painting with corse sand paper. next gesso the painting with good artist gesso...www.utrecht.com sells good gesso. next cover with two or three coats. no need to sand between coats. at some water to gesso to get pancake batter consistency. now your ready to paint with whatever you like. How to prepare an old painted canvas.?



The only thing you can do now is paint over with oils. You can't go over oils with acrylics -- the other way around; ok.


Here's your chance to try oils -- who knows you may like the medium.better than acrylics.


Just paint over the original picture with white oil paint (titanium white) and go for it. You may have to use more than one coat of the white depending on how much cover you want. Try a little thinner in the white for the first layer and then straight from the tube. Let dry( a few days), then go.


If you simply don't want to try oils, then I believe you are at a dead end.
You can paint oil over acrylics, but not acrylic over oil. It will crack.