My tank was in good nick also with no physical damage or rust. There was sludge inside & the most common suggestion I received was to use methylated spirits. Swishing it around seemed to dissolve a lot of the rubbish but the area I could see still wasn't smooth & shiny. Some small ball bearings would have probably cleaned the surface OK, but without being able to see around the baffle I couldn't be absolutely confident. I decided to take the tank to a radiator specialist as described below. It didn't need any repairs but the modest outlay to have it thoroughly cleaned & tested was worth it. Here's another thread on the subject. I used the same radiator shop for my fuel tank, heater & radiator.
http://forum.fefcholden.club/index.php?topic=26004.0