mechanically, and then use some good cleaner like Methyl Hydrate or brake cleaner. Apply an even bead and smooth it out (to about 1/16" - 3/32" thick) with your finger and put the covers on immediately. Tighten the bolts down just snug, maybe 2 or 3 pounds of torque and they won't leak. DON'T touch the bolts again after it cures contrary to what a few others have said.