I picked up a couple of Merc gas tanks off of Craigslist,$20.A terrible thing,Craigslist. One is in good shape, the other looks dark on the bottom and when I hook my finger through the cap hole I get rust dust from the top.Is there easy way to salvage it?
You can clean out the tank, if the rust has not penetrated the tank too much and created a leak. As a test I would fill the tank with water, I know... seems dumb, but its the cheapest thing to use to look for a leak. Let the tank sit, even over night on a newspaper, and check in the am for a leak. Given that there is no leak, you need to "get the rust out". First remove the fuel gauge and fuel pickup from the tank, if possible. Some tanks don't have the fuel gauge, and you cannot remove the pick up line.
The best method is to put something in the tank, l like to use round steel ball berrings. Then put the cap on, and shake the tank. Hold the tank up right, sideways, upside down, etc. You will have to do this quite a few times, unless you have access to an old paint shaker, etc. You can then use either compressed air, or a shop vac to clear out the "rust".
The more ball berrings you use, the quicker you will knock the rust off.
Once you feel confidant you have gotten ride of the vast majority of rust, put about a half gallon of gas, non mixed into the tank. Put the lid on, and swish it around. Siphon out the gas into a clear jar, and look for sediment, or
floating particulates. If you see lots, then your back to the ball berrings and shaking routine again. You can strain the gas, through a filter, and reuse it again for the next test. At some point you will get rid of the rust, or you could just throw the tank away, and curse craigslist.
I have had a lot of dirt bikes like this.. when I do it I use 3/4- crushed rock the sharp points make short work of rust... the take it to a radiator shop and have them do a cleaning on it and a seal... I did this and a few that had a few pin hole leaks and it seemed to seal them all