I have cleaned two tanks in the last 6 months, the 520 and Patrol. Here is my experience, based on input from jfa.series1 and a local radiator/fuel tank repair shop’s recommendations. 1) Use a Marine Clean (POR) product or similar degreaser (I used the inexpensive one that Wal-Mart sells.) This will cut the fuel varnish buildup. Add some small stones, old nuts & bolts, or chain link to the tank to agitate and break loose the buildup. 2) Collect liquid from draining tank for proper disposal. 3) Rinse with hot water. 4) Repeat Step 1 -3 5) Rinse with hot water. 6) Take to self-serve car wash and use their hose to spray high pressure degreaser then plain water. 7) You should be able to see through the fuel sending unit hole the inside bottom of the tank. If you cannot see clean metal repeat Step 1-5. If you see rust holes, buy a new tank. 8) At this point you can use the POR-15 tank coating. I have read extensively on-line that the coating may break down over several years and cause fuel system problems. (I chose to forgo the tank coating and primed and painted the tank.) 9) After completing the above, test the sending unit. Obtain a new sending unit gasket and reassemble. Keith