What can I use to get varnish out of my boats gas tank from old gas?

You must drain the tanks, and wash with "Diesel fuel", do not use "Gasoline" as it has a much lower "Flashpoint" and is dangerous to use due to a fire/explosion hazard. replace fuel filter. Gasoline Quality is at it's worst EVER, and they now put alcohol in gasoline, this is wonderful for attracting water in your fuel, as a matter of fact, I saw a bottle of fuel left out over night, the water, fuel, and alcohol seperated quite nicely so that you could see the layers. Use "Carbon Guard" fuel additive, it's cheap, and will prolong the life of your engine, I used it last year, and when I inspected the spark plugs, they were squeeky clean, the stuff works !

Replace the old gas with gasohol. It's 90% gas and 10% methanol. The alcohol is an excellent fuel system cleaner. It will quickly remove any gas varnish and moisture from your fuel system. Start with a fresh fuel filter. You will need to replace the fuel filter at least twice more before all the impurities are removed. Then install a fresh filter and your good for the season. A continued frequent use of gasohol will prevent the future buildup of varnish.

If you didn't store the boat for the winter with enough "stable" in the gas, then odds are your going to have to drain the old gas out of the tank and replace it with fresh gas.

Replace with plastic tanks, metal tanks are expensive, and a nightmare, plastic is SAFER, safety must be rule No. 1 on a boat, especially with Fuel, "cosmetics" is further down the list. Make sure that fuel tanks are well vented, some older boats have no "Blowers" to clear the gasoline fumes out of the bilge area, make sure there is a back fire flame arrestor on the motor if it is an "Inboard".

use a strong hot solution of a heavy duty cleaner called AquaKlean made by kbs coatings. Extended soaking may be needed to remove all the gum & varnish.

You can also use a pressure washer.

Try using Sea-Foam, it's a fuel stabilizer and fuel tank cleaner. You can find it at an Advanced Auto Parts Store

there is an acid you can use to clean gas tanks after you clean it real good you can see if you have any holes or leaks.

Get a bottle of STABLE. It is an additive that cleans it out.

A good filter is fail safe

Try rinsing it with carburetor and fuel injector cleaner-the stuff you buy in pint bottles(STP white).