“Sloshing Compound”

It was used in my tanks at one point, and the second time I filled with fuel after purchasing the plane I noticed stuff floating in the tanks. It flaked off in the size of note cards.
The tanks were removed and I attempted to clean out the compound, unsuccessfully. My A/P wouldn't try to take the tanks apart for fear he'd ruin them. Wound up having to send the tanks away to be cleaned out and rebuilt.

There seem to be many cases of sloshing compound coming loose from the walls and clogging up the plumbing. I would never use that stuff on my plane.

It is a sealer used to seal riveted fuel tanks. You pour it in the tank and slosh it around. You can still buy it on Aircraft Spruce. The old compound originally used in our Ercoupes starts to flake off after 70 years or so, reportedly resulting in the occasional leak and/or plugged fuel line.

OK; What is this ‘sloshing compound’? I keep hearing this, and can’t I mange what this would be.