I just went through this. I ended up using electrolysis by taking some old lawnmower blades and a charger hooked up for a couple days. Cleaned all the rust out. I should have but didn’t paint the inside as I was welding a patch in an area that had rusted through. Figured if the tank gave up the ghost I would just get a new one. Still working just fine. The pump was even locked up but I let it soak a while in a parts cleaner and a few taps with a hammer and she still is working.