I have heard of a couple methods for removing rust from rattling a chain/sand/rocks with diesel fuel in the tank to doing an acid treatment. Is this something I should attempt to do, pay for a professional repair, or just replace all together?
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Motorcycle folks seem to do this a lot, presumably because the gas tank is easy to remove and small enough to manhandle (personhandle?).
A Google search produces a lot of results for how to do this, and any of the ways you've heard will probably work well. Here's one detailed list: http://www.mopedarmy.com/wiki/Removing_rust_from_a_gas_tank
You have to decide what your expectations are. If you just want to remove a lot of rust and be done, use chain / rocks / screws / ice cubes. If you want a pristine gas tank that will resist rust for a decade or more, you'll want to get ALL the rust out and then coat the inside so it won't rust again. In that case, try POR-15.
I worked at a shop that repaired fuel tanks and this is what we did. No cutting corners, each step depends on the last.
The shop had a fancy caustic soda tub and some of the techs would call that an acid bath, but we used this on very few tanks. Mostly small, well constructed, steel motorcycle tanks. This is what we did for the other tanks that were not aluminum.
Some guys would thin the liner for the first coat, but that was not how I was taught. I think the results from chains and rocks would be too inconsistent. About once a year we would get a tank from some guy who put rocks in it and it takes extra labor to get that stuff out.
K.B.S coatings make a 3 step kit to clean blast and reseal tank.Involves tank removal (a compressor aids in one part of process-I used a hair dryer) and is suitable for cars and bikes.