CX500 GL500 650 Global

I'm in the process of cleaning a spare petrol/gas tank out ready to re-paint.

I removed the Petcock.

I then used some ball bearings cracked out of an old wheel bearing inside the tank to remove rust lakes by shaking them around the tank but be warned they are a PITA to get out once used because of the lips inside the tank orifices.Then I high pressure air blew out the tank and sealed up the hole with a small square of plastic shopping bag and masking tape and poured a bottle of White Vinegar in there and swilled around and left over night.The Acetic Acid in the vinegar helps neutralize the rust inside the tank.
Then next day emptied the Vinegar and air blew it out and then swilled some fresh petrol/gas into the tank and blew and left to dry again.You can speed up the tank drying as well by using a Hair-dryer to warm up the tank but keep well ventilated.

You can,if you wish,by commercial products to tank sealing like Por15

http://www.por15.com/POR-15/productinfo/1GB/

but I like to save money and go,"Low Tech" where possible.I've also sealed some small leaks in the tank inside using Polyester activated resin(Used to make fibreglass boats/car dent repairs).A couple of the small holes were present on the outside so I deliberately dented with a Ball Pain hammer and used a small square of Metal tape on them and then filled them with resin as well knowing that I will be using filler on these dents during preparation for re-painting.

Note: Many years ago I had a fibreglass petrol tank leak on an old Norton 500 and repaired it using Polyester resin with no problems.