For a cheap way to "shake" gravel or chain in a fuel tank. Use a small cement mixer. I made a couple L shaped brackets that are attached to the front edge of the drum on my little 3 cu ft mixer I got from one of the Home stores. I then placed a couple pieces of old heavy material like Blue jeans around the opening of the drum. Placed the Tank against this and using heavy bungie cords attached the tank to the drum. I put about a shovel full of crushed 3/8" open rock (no fines all rock is about 3/8") if you can't find this easy to make some from 3/4 minus just screen the rock with a 3/8" mess screen for the first step then do it again with a 1/2" screen you will end up with rock all about 3/8" with sharp edges.
Turn the mixer on and let it run for a hour. (remember to lube the mixer. I then turn the tank 90 degrees and let it run another hour.
Came out looking almost sand blasted but shinier.