So on my '73 Scamper I started to clean my 3HP Tecumseh up to get it ready for paint and there is this film like stuff inside the tank from the old gas. Anyone know any good ways to get rid of it? (Note: it is a plastic tank)
The first thing I can say is: be patient. It is a labor intensive and time consuming process. I completely removed 50+ years of rust on two tanks this past summer using a product called Metal Rescue, it is a chelating chemical NOT an acid, which is poured undiluted into the tank and literally "absorbs" the rust out of the metal on a molecular level. It is impossible to leave the Metal Rescue setting on the metal too long. It simply will cause the metal to take on a protective black oxide coating if the piece you are working on sits in it longer than it takes to remove the rust. I had excellent results using it as I followed the directions, the most imperative thing is to keep the metal rescue hot, as that is the only way it will work, in fact I recommend heating it up on the stove in a old pot for 10 minutes or so before you use it so it is hot. It should be noted that before using the Metal Rescue I placed a bunch of old nuts, bolts and brass scrap into the tank and shook it around to loosen the bulk of rust, then cleaned the tank with pinesol diluted in water. The piece you are working on must be washed with fresh metal rescue after it has soaked, do not wash the piece in water after using the metal rescue.
I also keep my tanks clear full all the time (especially during storage) to avoid condensation at the top of the tanks. Another reason of keeping the tanks clear full of gas always is to take advantage of the corrosion inhibiting effect of gasoline.