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?
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.
- Emptied and pressure washed.
- Dried with a large butane torch on low setting, wire brush mounting areas, around seams or other likely areas.
- By now you should have an idea if the tank is repairable. If it is, blast with glass beads. Make sure the tank can withstand the glass beads - a healthy tank can.
- We repaired pinhole leaks, pitting from rust, reinforced seams, etc.. with a lead solder. Max size of a single hole... about a penny or dime in diameter. Use some small pieces of metal for reinforcing, if needed.
- Clean flux off and prep for liner by sealing any vents or tubes
- place tank in the water test tub. Pressure test with just a pound or two of air, any more than that could misshape the tank.
- Start liner process. We used Red Kote and air dried... sometimes it would take 3-5 days per tank. Very simple process, pour just enough liner in, to coat the side of the tank and let dry. Turn and repeat every few hours and take your time - Red Kote is thick and slow moving.
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.