Last fall I took the tank out of my '67 Monaco. The car had been in storage for 20 years and there was a couple gallons of "turpentine" in the tank, but it obviously had leaked more out over that time, a thick sludge of black tar on the floor under the tank. God knows what they put in gasoline to do this.
I took a small grinder to the J-bolts (they're easy enough to make out of a pair of regular bolts).
There was some sort of gritty stuff in the tank, like sand or dirt, a lot of it. I drained the tank, but didn't really flush it. I set the tank outside for the winter, protected from direct exposure but still able to air out. Took the sender out, it looked like it came from the titanic. I had a new one, bought 20+ years ago.
At the time I called a rad shop about having them work on the tank, they said bring it around. I didn't. I called back about a month ago, they say they don't really fix these old steel car tanks any more. Great.
So I make a cover plate to seal the sender hole, and then begin to de-rust the inside of the tank with vinegar and electrolysis.
About 2 gallons of 10% cleaning vinegar, rotate the tank to get the sides, ends, top and bottom, about 8 hours each session, leave the vinegar in there the whole time, about 2 weeks I guess. Have to watch for leaks from the holes. Using a lab or bench power supply, set the current sometimes at 1 amp, sometimes at 6 amps, usually to get 60 watts of power flowing through the vinegar. It gets warm.
Then I start flushing it with water. Flush all that sand and grit out. Let it dry. This is what it looks like.
I should check this with a magnet. I can't remember if back in '87 when I worked on this car if I used a tank coating on the inside. This might be what's left of it. Either that, or this precipitated out of the gasoline over the past 20/30 years.
This is what the inside looks like now.
The de-rusting has created more pinholes. I've tried soldering some pin-hole areas but I can't get the solder to flow like I want to cover these areas smoothly. I'm going to use fine copper screening soaked with solder like a patch to cover these areas.