When you paint over an old painting you run the risk that what was underneath will show through, as gen2 intimates. either as texture, or as color if you're using transparent colors or glazing. It's important, in re-using canvas to know what medium was used. You can use oil over acrylic but not acrylic over oil.
If it's oil and has been "retired" for several years, it should be thoroughly dried and the cracking should be ended. I would sand it with very fine sandpaper to remove unevensess and thorughly remove all dust. Cover the painting with two or three coats of titanium white, and proceed.
If the original medium was acrylic, just wipe thorougly to removed dust, gesso as usual and proceed.
One thing you could consider is to remove the canvas from the stretcher and turn it inside out presenting you with a new canvas...
Excuse me while I diverge for just a moment...
Q... when are going to see a report of the latest gastronomical outing to the continent?
Thanks, elemenope...