Sealing an oil pan leak with JB Weld is going to be difficult at best- - - - -getting the pan clean and dry enough for the epoxy to get a good seal is a real challenge. Are you dealing with a crack, a rust pinhole, or what? Rust holes in particular are problematic, as the hole on the outside is usually the "tip of the iceberg" with a much larger thin spot on the inside. Remove the pan and get it completely degreased inside and out, and maybe get the area around the leak beadblasted to give the metal some "tooth" for the epoxy to grab onto.
Jerry
