How do I tell which gaskets I have?
It will depend on the age and source of the cans, especially if they were actual US military issue.
Recent commercial issue cans can be easier to deal with if they are color coded for fuel type. Either the entire can or at least the retention strap on the cap will have a color related to fuel type.
For the last two years Viton gaskets have been a gray color and standard rubber have been black colored. There are no part numbers or markings on theses two types. If you bought old stock this color scheme might not be true even for color coded cans.
Gaskets on old stock can be black for both Viton and standard rubber, even if the can or strap are color coded. If you have a black gasket on a can that you think is for gasoline you have to remove the gasket and inspect the gasket's edge. Black-colored Viton gaskets will have a small notch that is colored yellow.
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Basically without a history of your can there isn't a good way to tell if you have viton or not. It comes down to risk assessment. If you roll the dice and its viton you saved $8/can. If you roll the dice and its not viton you will be replacing the flange ~$15 and buying a new gasket ($8) anyway for an additional cost per can of $23. For me I'd just spend the $8 upfront and have peace of mind but its really up to you. You could always IM @vatopa and he will be happy to chat with you about them.
If you have a can with paint on it this is how I ended up getting it off. I used "Goo Gone" and those flat abrasive green kitchen "scrubbies". Cover one side or section of side with goo gone and let it sit for 3-5 minuets. This softens the paint. After it is soft use a paper towel to wipe off the loose paint. Re-coat the can with goo gone and then start scrubbing with the green scrubby. Do not let an area get 'dry'. Keep reapplying the goo gone while you are working. It gets the paint softer and softer. The real secret is not to be stingy with the green scrubby. I used 4x4 sections and flip over once they are full of paint. Just keep tossing them! Don't try and get another 20 scrubs out. Trust me it will be WAY faster if you don't try and stretch the life of the scrubby. I had seriously painted cans. I spent about one hour cleaning each one on average.
Mine were almost spotless in the inside but the couple that had old diesel grunge in them I just rinsed with water. You want to get ALL the water out when you are done. I shook them empty, blew out the vent tube with compressed air and then ventilated the cans with a fan and plastic tube for a full week to ensure the water was all evaporated.
FUEL CAN ORACLE
Lots more information at the above link. It deals with all kinds of cans including MFCs.