One more question, McDave. Is it necessary to open rust bubbles that have formed under the paint, or will the oxalic acid find its way to the source, loosen it, and cause the paint to flake off easily when brushed? There's quite a bit of subterranean surface rust (for lack of a better term) that I would rather not attack with a wire brush or razor if the acid bath will do a cleaner job. Thanks for all your help.