This is a cut & paste as it was typed out by Scott Harms Smith.. He doesn't give a ratio but tells you what consistency to aim for... I would have offered this same info to a VW owner, just didn't see his post...


Start with 100% pure Acetone, buy a gallon. 
Pour an ounce or so into a glass jar (small open mouth jar). Take a scrap grill and using something sharp, scrape off peelings (like pencil shavings size) into the jar of acetone. Put allot of shavings in, the acetone will gradually melt the plastic into a goo. Add enough plastic until the goo reaches toothpaste consistency. Once you have a good batch melted put the lid on the jar tightly and set it aside. Now, prep your grill as normal, V out cracks on both sides, etc. Use clamps near the joint to align the two sides flush and parallel. Smear on some plastic goo, leave it crowned up over the crack like a regular weld looks, let it cure overnight. Sand to flush the next day, fill any pinholes with JB Weld. The crack will practically disappear at this point, paint as normal