You can always try "evaporust" you can find it at home depot or even harbor freight. I've used it with great success on lightly rusted tanks, make sure you fill it up right away to prevent further rusting though. Also it helps to throw some metal nuts in there to knock around the rust, cheapest way I can think of unless you have the materials to use electrolysis.
Getting the tank clean is the first thing to do,otherwise the rust will end up coming through your Kreem or whatever you use to seal it. The electrolysis method is a thorough process, but time consuming.It does a great job though. Only have to keep changing the electrod that collect's all the rust.
Travis' post show's how it's done.
Bowl Works from the dollar store, leave overnight and clean. If you like dancing go with the dry wall screw method. I like Caswell platings sealer over por. POR is nice good but leave Kreem on the shelf.
Hey there. Try removing the fuel valve(s). dump a couple of handfuls of dime sized nuts in the tank, fill it half full with warm dish water, close the cap and shake till your arms are weak and after you've rested go ahead and shake it some more! The size of the nuts are important cuz theres lip under the fill hole and you won't be able to get them out through there. Shake them out the petcock ports. After you've drained and rinsed the tank dump a bottle of drug store alcohol in the tank and shake it some more, this will absorb any remaining water in the tank. You'll have to fab some kind of stopper for the petcoks port for the "shake" cycle. This will get rid of alot of the rust, maybe not all of it and it wont keep it from coming back over time. If you're going to go through all the effort you might as well do the final step, spend the 50 and seal the tank with one of the kits. Hope this helps!
Most tank sealer kits should include some etching acid to remove the rust.I've used Kreem with great results in several tank repairs that required fiber glass on the bottoms and tunnel areas to prevent leakage on obsolete BMW tanks.
I have two tanks. So I thought of trying the electrolosis on the rusty one. I first plugged up the petcocks inlets then filled the tank with water. Add 1/4 cup of baking soda from the kitchen and mixed it well. I cut up an old tire tube to stop any grounding around the filler cap. Next, place a metal rod in the tank making sure it does not ground in the tank.
Hook up a battery charger Positive to the rod and negative to the tank.
I left the charger on for 6/7 hours. The rod i placed in the tank was almost completely gone so was the rust. just a few spots left.
^Agree. If you want, you can finalize with shaking a foot or two of chain in it. You could probably do the final rinse with alcohol to prevent any "flash rusting". Or maybe water soluble oil, like bacon grease
If you have a bit of time and depending upon how much rust.
I have used a product called Metal Restore to remove the rust.
It only dissolves the rust does not attack base metal or paint and can be rinsed away with water.
Once the tank is clean use Red Kote tank sealer.
I have used the Kreem tank sealer, this kit contains a corrosive acid, nasty for paint and hard on base metal.
The Por 15, is just a glorified paint, it becomes hard and in one word SUCKS.