I found the best way to make the sludge as thick as I could and then paste it on rather than trying to pour it. The more thinners is in it, the longer it will take to set.
Then carefully using a hair dryer to heat it up, be careful though cause if you get it too hot, you'll get loads of little air bubbles in the plastic that expand and make it look really bad... I know cause I did it
Warm it up gently, then leave it somewhere warm and dry like a hot water tank closit/airing cupboard over night. And DO NOT under any circumstances, try to sand the gunk until it is absolutely 100% cured and hard, otherwise it just drags when you sand it, and clogs up your paper, and again looks sh1t.
The ABS gunk method is very affective, last years, and looks extremely well once done, its just very very time consuming
Lots of patience is required!