MEK (Methyl Ethyl Ketone) works much better then Acetone.
I cut up 1/4" X 1/2" chunks of ABS and fill a pill bottle about half full with ABS pieces and pour in enough MEK to cover the ABS and let it soak for a few days, stir it up occasionally. After the ABS has melted leave the container open and let the MEK evaporate off until you have the consistency you want or add MEK to thin it up.
I have been using this to fill fill hole and repair cracks, for cracks I drill a hole at the end of the crack to stop it from progressing and dig out the crack on both sides and fill it with my MEK concoction. Takes two or three applications as it shrinks when it dries and you can shape it with a razor blade the next day as it is still fairly soft, it has been 40 degrees around here and it take about 5 to 7 day to complete dry.
I discovered by accident:
I was making up a batch of the MEK, ABS concoction and mistakenly used Xylol instead of MEK and the next day I discovered the ABS chunks were clean soft and pliable but had not melted at all.
A few weeks later I was working on my A/C shroud and where it attaches around the fan it was brittle and dis formed, I could here it cracking while I tried to screw it down. Remembering my Xylol, ABS mistake from earlier so I soaked a rag in Xylol and draped it over the dis formed area with some Saran wrap over it to keep it from evaporating. I keep the rag wet with Xylol for abut 30 hours, after this time I was able to clamp and reform the ABS where needed, I left the clamps on for two days. It has been a week sense I did that, I am going to let it dry another week and if it still looks good I will do three more areas that need attending to.