off point, kinda- I had a 1965 Honda 50 (didn't everyone?). Used steady until stored. Gas left in tank, no proper storage prep. A guy wanted it back about 8 years ago. I dug it out, tires flat, tank rusted and floating in the old gas. Put the key in, primed it, started on second kick. Drove it around the yard!

 

When I got the tank the previous owner had laid it down in his yard.. when it went down the drag bars he had on it hit the tank.. causing a golf ball size dent in the tank.. when this happened apparently the rubber liner in the tank cracked chipped off leaving little plastic flakes and bare metal exposed and over time it rusted.. I know this because I had to tighten the pep-**** on the tank once and decided that while I was in there I was going to inspect it.. and glad I did.. there were paint chips all over the filter.. im honestly surprised the thing ran.

To get the flakes out I ran the tank till it was almost out of fuel.. drained the rest, pulled the tank and used gas to swish it around to try to clean it out.. poured it out.. and repeated the process until I got nothing out.. it took about 10 time or so..

and the only reason that I know the extent of damage on the tank is because I was the one that repaired it..

 

Time varies by if the protectant inside the tank has started peeling off, it gets hit with gas nozzles and starts goming away sometimes, then when left empty condesation forms and starts to rust the tank. Best to store your bike with a full tank off hi octane and a bottle of stabill is what I've been taught.

You can recoat the inside, or have it recoated, but that involves stripping the inside, getting the existing rust off or changing it from iron oxide to black oxide, or another element chemically, then recoating the tank.

 

They do sell the coating so that you can do it yourself but I've heard how messy it is.. and as far as storing my bike.. it only sits up maybe a month or 2.. even in that time.. if its warm enough to stand it I try to ride..