Is there any way you can keep on painting on an oil painting using acrylics? (Other than starting at a new painting.)
Maybe you can add something to the acrylic paint?
(I know they don't mix well, that's not the question.)
As Catija implied, if your oil painting is less than 6 months old you shouldn’t do this. And if the oil paint is impasto (very thick) you should wait up to 18 months before varnishing. A very bad technique that would work is to varnish with a solvent-based varnish, let cure for 72 hours and then overcoat the varnish with a water-based varnish. Then you could theoretically paint with acrylics on top of this, but if your substrate is canvas I can promise you that you will sooner or later see cracking and flaking - especially if the “final” hanging place has humidity swings.
One last reason why this is not a good approach is that conservators will find it pretty much impossible to restore this kind of work. Basically you are dooming the painting to be more temporary than necessary.
Why don’t you just use oils?
You should never paint over oils with acrylics. The reason for this is that acrylics are essentially a plastic and not breathable. Oils, however, release gases over time. If you paint acrylics on top of oil, the released gases will cause the acrylic paint layer to crack and flake off as the gases try to escape. This result will be amplified in fresher oil paint.
You can, however, paint oil over oil and oil over acrylic.
I’m an experimental artist and have had success with this technique plenty of times. I use oil and thin in turpentine, not too much though. Then allow 24 hours of dry time then apply an impasto layer of acrylic ; the result is awesome when it starts to crack. I sealed my work with resin so no flaking happens and yes the resin will make the colors and other subtle layers pop. Have fun with painting, experiment with different mediums... see what happens. ~ finch
Not really unless you don't care about the longevity of your painting. Oil never truly dries (it's sort of like glass which is actually a liquid).
With acrylic, an actual chemical change takes place as the polymer chains link up. The issue is that the acrylic will die and not be able to stick to the oil. It is not the acrylics' fault but the oil. Of course the longer you let the oil dry, the more solid the surface will become and the longer the acrylic will "stick" to it. Maybe after a century it might work. The issue is that there is nothing you can put on top of the oil to solve this problem because it too will have the same issues.
Now you might be able to use certain types of modified oils when painting but you already have the painting. Even if you painted over them you will have the problem. Maybe if you bake the painting that might help speed up the drying.
Ultimately though you can't win in any practical way.
You could paint epoxy over the entire painting so that even if the oil wasn't completely dry the epoxy layer would be one solid layer (assuming it's thick enough) which might have enough adhesion not to fall off... but then your painting will be extremely rigid.
Ultimately, you can do a lot of things but you are risking ruining the painting and it's simply not worth it. Just move on and don't waste any more time with it.
You do have water soluble oil paints but you are always going to have the same problem unless the original painting used these.