Technique used for painting over old paintings

I assume when you mean “used” you mean they have paint on them? If so it is not advisable to reuse them although this can depend on the materials and technique that you are using. There many examples of oil paintings that have been painted over by the original artist (Picasso is one such example) or by another hand and while some have survived fairly well others have not. Fresh paint needs to be applied over an absorbent ground or over paint layers that are somewhat lean (remember the “fat over lean” rule). When paint has had some time to cure and harden over a period of time (as in years) it no longer serves as a suitable surface to continue painting on (plus you risk dealing with flaking, cracking, and delamination of the top layers of paint later on). One can gently “rough up” the surface using a fine grade of sandpaper to restore some tooth HOWEVER you should take care to wear proper protection to avoid breathing in airborne particulates, particularly if you use pigments that are known to be fairly toxic. You can wipe down the surface of your painting using mineral spirits to remove any additional pigment or residue on the surface. If your paintings possess fairly thick grounds and/or paint layers you should probably just buy new canvas.

A couple of additional comments….just because someone may not have experienced problems painting over old paintings does not mean it will not occur in the future. Some examples that can be found in museums have withstood the test of time thanks to conservation efforts…sometimes problems will not arise until after an artist has left this earth. You can of course use such canvas for experimental purposes or as Matthew suggests paint on the reverse. Finally, those that feel their works only need to survive while they are alive are doing themselves a disservice as well as others (this was stated on another forum recently). This is especially true if you sell your works or even give them to friends and loved ones. If you paint for the love of it that is wonderful…but you can also do so while exercising best painting practices.

If your paint and ground layers are thin and lean it may be alright…again it is best to not do so but you can probably get away with it if your layers are truly thin and absorbent. Keep in mind you may have some interesting pentimenti form later on down the line but that may not be something that troubles you.

Sanding would likely be effective unless the paint is overly fat and vehicular. It may be better to use a scrapper like those used in the 19th century to reduce very fat layers before painting. You mention old dried paint films, turpentine should not effect a old and properly bound oil paint film. Deglossers, which is really what you are describing, are not suggested in fine art oil painting. The acetone,, if it is effective, would destroy a portion of the upper skin of the painting, it does not redissolve it (like it would with gouache or a resin paint like Magna). This could create an unstable interlayer in the painting. As far as paint so old that overpainting becomes difficult, I would suggest light sanding using all appropriate precautions and painting into a couch of oil (see the resources section on the safe use of the oiling out technique) rather than deglossing with highly polar solvents.