this is taken from the FISD tutorials:
Step 1: Cut ABS Strips
Cut thin, short strips of matching ABS into a (clean) cup or similar. A glass vessel is preferred! Don't go pouring acetone into a plastic cup.
Step 2: Mix with Acetone
Carefully pour acetone so the fluid covers the ABS shards. Go easy on the acetone - add a little at a lime instead of drowning the ABS shards. We're talking really small amounts of the stuff here - a couple of thimbles worth of shards and acetone.
Step 3: Wait
Depending on the thickness of the ABS pieces, acetone dissolvation time might vary. Check progress every now and then, stir a little with a (clean) rod or stick. Apply more acetone if plastic won't dissolve in a satisfactory way. Go easy on the acetone though - you want a runny, but not watery substance
Step 4: Apply Paste
When the acetone/ABS concoction is a bit on the runny side, begin applying to damaged area on armour or helmet. Easy does it - make sure you fill the gouge properly.
Step 5: Let Dry
When happy with the results, leave alone and let the acetone dissolve fully. ABS hardens and fuses with your armor/helmet. Regarding drying time, it depends on amount of ABS goo used and size of damaged area. YMMW. If your ABS milk is runny enough, it will just fill the damaged area - no need to sand. Still, if you get a bump, sand it down with fine-grit sandpaper, buff out the rest with Novus, and you're done!