Slosh
Slosh explained
Once a widely accepted practice for sealing tanks, this milky white coating found in many early RV tanks is one of the most dangerous accidents waiting to happen. This technique for sealing tanks consisted of pouring several cups of sealing liquid into the tank and "sloshing" the fluid around to coat the inner walls. This coating, once dried, seals the joints and rivets from the inside of the tank. When the slosh solution has something relatively rough to adhere to, such as a rivet or crack, it performs nicely. The problem lies in it's inability to properly adhere to smooth surfaces such as the inside of the tank skin or the rear baffle. The white, rubbery coating peels off in large sheets sometimes as big as several square inches.
Imagine a piece of this material wrapping itself around your fuel pickup on climb out. If you know or suspect that you have this material in your tanks, you need to determine its condition. Remove your fuel cap and peer inside your tank with a flash light (no matches). If you see any particles floating around inside your tank, you must address this problem. By far the easiest and best way to repair this problem is to replace the tanks. The tanks can be cleaned out, but it is difficult, very time consuming, and likely to damage your tanks due to the handling necessary and the strength of the chemicals being used. Click here to read our article used as the source for the column published in Van's Aircraft's periodical, The RVator