I just went through a complete tank cleaning process as I recently found a VERY gummy black tar like substance completely blocking the tank strainer mesh even though I had used a BioCide plus other diesel additive treatments in fuel. In addition to cleaning I was planning on lining the inside of tank with a tank coating material. So this is what I did in hopes of getting it REALLY REALLY clean:
1. Remove all tank fittings except fuel cap and then plug them closed; you will need (4) 1/4" NPT pipe plugs, (1) 1-1/4" NPT pipe plug, (1) 1-1/2" NPT pipe plug; black iron fittings work fine for this purpose.
2. I used (1) gallon of Krud Cutter (advertised to cut tar and grease even on BBQ grills) and filled up with hot water and shook up and let sit 24 hrs. I put the tank cap in the closed fording position and also turned tank upside down to make sure top of tank would get the treatment when I repositioned tank every few hours. You could substitute your favorite degreaser cleaner (ex: TSP solution, or whatever works)
3. I then removed all pipe plugs and drained out KC solution then used a pressure washer with one of those rotating tips that is used to clean driveways, decks, etc. It rotates at a very high RPM and uses a small jet to produce a very high powered effect. After removing the fuel strainer I was able to slide the wand tip through the openings of the cage that holds the fuel strainer in place. It blasted a lot of gooey crud away. I was also able to fit the wand tip down through the removed fuel gauge opening and angle it all around.
4. After getting a lot of crud out I did another cleaning with hot water with some dawn poured in the tank and then blasted it again with pressure washer. Lots & lots of foaming suds! Then rinse well.
5. I then filled tank about 1/4 full of coarse river sand (the kind with large grains of sand and small buckshot size gravel and then strapped it to the rear wheel of my tractor and drove it around our property for about 1/2 hour then emptied out sand. Bottom and sides of tank were all shiny and silver. The sand I emptied out smelled of diesel, so even after all the above washing/blasting there was still stuff the sand rubbed off. The sand treatment got a lot of stuff that everything else missed. I then did another final power washer and rinsed well.
6. Re-plugged tank and I then poured in 2 gal of water and added (1) gal of Phosphoric Acid to make a 33% solution (25-50% solution is recommended for neutralizing rust) and let that soak for 24 hours (rotating tank with solution inside on all 6 sides every few hours to insure the solution worked everywhere. It turns any remaining surface rust into a black rust proof coating (forget the name of chemical process). Rust on underside of fuel cap turned black.
7. Rinse, rinse and rinse and do a final rinse with acetone to get all water out.
I was planning on coating the inside of tank with a product called Red Coat. The instructions cautioned when using this product on a diesel tank you have to insure ALL DIESEL RESIDUE is gone. The manufacture mentions that Diesel stored overtime leaves a paraffin coating on metal and the only way to remove it is by sand blasting. So I had hoped my rotating power wash wand in combination with the rotating river sand treatment would scrape off any gummy paraffin residue. Manufacture warned that if you use their product on a diesel tank and don't get all the residue off the Red Coat material will NOT adhere properly and will eventually release over time and can cause problems with fuel system (pumps, injection pump, injectors) so they said "use at your own risk" on diesel tanks! Well even after all that cleaning and seeing a clean silver bottom and sides I stuck my finger through the strainer cage and was able to touch the underside of the tank top and it was STILL GUMMY TACKY feeling. I tried using mineral spirits and even acetone and nothing would touch the gummy stuff (paraffin) still adhering to underside of tank top. So I had to abandon any thought of lining the tank with Red Coat. Regardless, I had a really clean tank but did not want to risk having the plastic liner coating detach and be fighting fuel contamination problems down the road. So I just cleaned up all the fittings (tank gauge, aux fuel level switch using emery cloth and mineral spirits) and put it back together and called it good. I will probably have to do this every 4 years or so due to our wet winter climate and the fact our diesel has biodiesel (B5-B20) in it. I recall reading somewhere that Biodiesel can become corrosive when condensation gets mixed in with fuel over time no matter what fuel additives and stabilizers are used.
