Mav70, the issues you're running into remind me of what I'm going through with my dads truck. 64 c10 LWB 292 4 speed granny low. His gas tank looked like something out of cave diving documentary. Stuff growing out of the sides and bottom, yellow, varnished, nasty stuff. We had a spare gas tank we got in a craigslist purchase of a different 292. That block turned out to be cracked, but we got a lot of parts. Well the tank they gave us was practically spotless inside. Save for some "white rust" (yes, it's a thing google it.) Not wanting to risk clogging up the new sending unit sock I wanted to clean out the tank somehow. Here's the best advice I could glean from YouTube and the internet:theres quite a few different methods for cleaning a tank. Best solution obviously is to replace the tank and sending unit. If that's not in your budget then you have to get the bad stuff out and ideally prevent it from coming back also, neutralizing whatever method you decide on to prevent further damage. Being that the tank we have only had some white powdery stuff inside I didnt want to get to aggressive with it. Some type of media would be best for heavy sludge/rust/junk. Like:Nuts,Chain,BB's,Pea gravel, etc. What you want to ensure is that whatever you put in, comes back out. So count all of the pieces of whatever you use to ensure they are all accounted for when you dump them out. I ultimately went with apple cider vinegar for cleaning my tank. Picked up a gallon at the local supermarket for about $5. Dont get the organic stuff costs almost 4 times as much. I poured in about a half gallon and gave it a good shaking and swishing around. Then I played it on each different side as best I could for a few hours each. Then poured out the liquid. It looked ok maybe a little tinted. Then I washed it out with the garden hose. Then heated a gallon of distilled water on the stove in a big pot to almost boiling and added about half a box of baking soda and stirred it real good. Then poured that in about a quart at a time and shook it all around in the tank. Emptying the tank each time. One last rinse with a gallon of distilled water from the jug and put my air nozzle inside till it looked dry. Sealed it back up until I was ready to install it in the truck. When I unsealed it, it was still dry, no rust and no more white stuff.
