The title says it all.
My girlfriend bought a 1985 Nighthawk 450 as her first bike with only 8000 Kms on the clock. However the loser who owned it bought it new then drove it for 2 years then let it park for 25 years with half a tank of gas
Fortunately before he stored it he drained the floats so the bike runs fine from an auxiliary tank. But of course the fuel tank was rusted to phuck. I mean this is literally the worst tank I've seen in 25 years of riding, not only was it rusty but there was 25 years worth of dead gas varnish as well. So I went and bought some very pricey Evapo-rust stuff that worked ok, but I couldn't afford to fill it up to the brim as it would have cost us $140 bucks just to do so. So after a little research I heard that you can put vinegar in there and it will clean it up. So I figured what the hell at $2 for 4L it would only cost $6 to fill the tank. So I filled it up to the brim with the vinegar and left it for 2 days. Holy F@ck batman! it came out like new inside! I'm doing another treatment now for a couple of days just to be sure all the rust is out of there. I've bought some baking soda to throw in there once the vinegar is drained to neutralize the acid and stop flash rusting. Give it a try it's one hell of a cheap fix. Just be careful not to leave the vinegar on the paint as it will eventually eat it.
What you need:
A shitload of vinegar(enough to fill right to the brim)
baking soda(to neutralize the acid)
patience(it takes a day or two)
WD-40(if you're not going to mount the tank right away as it will protect the metal)