A FAQ here often crops up and it is about asking how you prepare an old canvas for repainting. I just did this process this morning and so I thoughy I would not only show it but then I would have it in the bank to link to in the future rather than explain it over and over again.
This is a process I learned about 35 years ago and have never seen any problem, I know of a professional artist who used it throughout his career in the twentieth century without issue, still I disclaim any guarantees whatsoever.
First I make and keep on hand this emulsion solution:
1/3 water, 1/3 denatured alcohol, 1/3 gum spirits of turpentine.
I use distilled water and hardware store alcohol and turp.
The water/alcohol mix but the turp will separate upon standing.
It has very strong solvent dissolving power.
I shake well before using and it makes a milky looking emulsion.
It is particularly strong and pungent and I only work with it in a well ventilated area like outside.
First I prep the painting by lightly abrading with a green scotchbrite pad.
Then I apply a good amount of the emulsion and spread it and let it soak, in this example I am showing, overnight, but usually a half hour is ok. This gives it time to soften the paint surface for the next step.
I wipe that off and reapply but this time I really scrub it using a rough old towel or rag.
Here is a painting that was a quick 9x12 plein air done last spring that I did on a nice linen/hardboard panel. I really wasn't happy with it and not real interested in fixing it, I can repaint the scene next spring if I want. But I wanted that nice panel for usage. (I can be ruthless like that)
So here is the painting before and after:
Now I let this dry very well and I will probably paint right over it but if I wanted to, I could retone it with a coat of oil paint of any color.
If I have a painting that has very thick texture with high ridges of paint, I write that one off and scrap it. I know from experience that it is impossible to get the surface like I want it being a little particular in that regard.
That is it!