I've done this with two boats, Catalina
28 and now our 36. I took them both to a rad shop that does this work. They clean it using a variety of wands made to get in through the fuel gauge hole, the filler, vent and fuel line holes and whatever other holes exist in the tank. They put a new gasket
on the fuel gauge sender when done. They use mechanics mirrors and small wand mounted lights to see inside the tank. Both tanks were spotless when done. Cost $75. Judging by the fuel that came out of the bottom of these tanks when I pulled and drained them, it was money
very well spent. The fuel tanks are so easy to remove and the cost so minimal I intend to do this on a 5 year basis, I prefer to be proactive about maintenance
rather than waiting for the engine
to die and discovering that a problem exists. I will wager there is a rad shop somewhere in your area that does this, or a company that specializes in fuel tanks given that you are near a bigger center than the boonies where we live.