If you want to line the tank look online for the kits Randall mentioned. Having used a couple I will suggest the POR-15 motorcycle tank lining kit. Though it says it is for motorcycles, there is enough material in it to coat up to about a 10 gallon tank.
That being said, if the tank doesn't have any holes I would not line it. Using the liner kit may create more problems than it solves. Your CLR treatment is good. If you want to be doubly sure, clean the tank first with a strong caustic cleaner (I like ZEP Industrial degreaser concentrate from the home center). Add some old nuts and bolts to the inside of the tank and slosh it around every so often. That will remove gum and varnish from inside the tank. After several hours of sloshing the caustic cleaner around, rinse the tank with water and repeat with CLR or phosphoric acid. After several hours, fill the tank with water (leaving the CLR or acid in the tank) and allow it to sit for a day (full day) then drain and dry the tank. The CLR or phosphoric acid will dissolve or loosen the rust scale. Don't forget to take the the lose nuts and bolts out of the tank.