That ol' demon water, collecting within the fuel tank, has brought grief to many. Condensation due to temperature swings, long-term storage with partial fuel, a neighborhood gas pump without a good in-line filter and more.
Even worse for the R65 with a single petcock, the right-hand side of the tank never drains fuel or water, unless the owner does an annual dump n' rinse. The welded tank seams are prime rust country.
A tank liner treatment may work, depending on the extent of the rust. Most good liner kits include a rust removal/neutralizer chemistry. Works fairly well with pin holes but anything larger will likely need to be brazed/welded. A wise welder would want the tank completely flushed and free of gasoline fumes. I saw a 'tube' video where a guy ran a gas welder flame over the open fuel cap and Poof! Nice sized flame erupted. Then he went on to weld up the tank.
A decent used tank, inspected for no rust, might end up being more economical. A repaired tank or its replacement would both need fresh paint so that may not factor into a decision.
I just bought a pair of gently used BMW handlebars from Larry Stonestreet; owner, operator and janitor of Cali-based Stoner's BMW Stash. Hit him up for a used R65 tank. He's an Airhead good guy. No web site but email works fine. <ca.stoner@yahoo.com>
