i have several unfinished paintings from years ago that i would like to finish now; unfortunately i don't remember if i used oil paint or acrylic paint on the pieces - is there a way to tell if it's oil or acrylic paint on a canvas? does it matter if i cover oil paint with acrylic or acrylic paint with oil?
Alcohol is a solvent for acrylic paint. If you put a few drops of alcohol on a corner of the painting then use a Q-tip and rub it, the paint starts to dissolve it's acrylic. Or sand a section a bit and use the alcohol on that if nothing seems to happen. If you have a painting thats oil, you'll see no color comes off on the Q-tip.
I would stick to the paint you used for best adhesion.
Old oil paintings get a hard surface and you can lightly sand the surface for a good bond with the new paint. Acrylic will stick to anything.