Why the sealing if it's not leaking? It just needs to be cleaned.
Go to Walmart and get a gallon of the cleaner that comes in the purple bottle. Mix it full strength, fill the fuel tank and let it sit or a day or two, swishing or shaking it. Dump the tank and take it to the car wash and do a couple of cycles fr pressure washing, take it home, let dry, and reinstall. I did this many times with diesel tanks in old sailboats and it gets them spotless. May as well replace the pickup tube also.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner