NOTE: When a cylinder bottle is rusty to a point where gas leaks, however small, do not attempt to repair. It can be dangerous and must be destroyed. Once a cylinder bottle is damaged, it is damaged. No repairs or patches should be done.
So, what should you do to your rusty gas cylinder bottles at the corner of your store? Should you either give it to a professional recycling company or just do a spray on it yourself, but understand strictly safe environment. I will recommend you take any rusty gas cylinders out for recycling. It is the best and safe way.
However, if you still think that rusty gas cylinders can still be useful, you can respray them. I recommend the "Best Textured Spray Paint for Metal— Rust-Oleum Textured". It is fast dry and used mainly for interior - exterior purpose. It is also anti-rust too.
Here is one I found at the local store in my area - Visa Aerosol Spray Paint. Not that popular thou, can't find online also. But, it is a great Spray Paint. Not long lasting, but it does do the job. Anti-rust also, however, you will need 2 or more bottles for 1 gas cylinder spray.
To start, if your gas cylinder bottle outter body is completely rusted, you can use a scrape or a "sand paper" together to smooth up the areas, gently without knocking on denting hole in anyway; paying great attention to the position of the vent hole. With the aid of a paper-tape, cover up the gas bottle valve stem. Spray in a up down direction at correct angle. Use a steady, sweeping motion.
To avoid runs or drips, it's best to apply several thin coast rather than one heavy coat.