36 or 40 grit are nice and abusive, i use those for grinding or flatting welds back and softening or shaping corners. 60 grit is less abrasive, 80 even less and if i remember 120 grit gets a lovely finish. There are speacial discs for removal of rust or paint, i know weld equip sells them, if not i would use an 80 grit disc.
I like 120 for paint and rust removal and 80 for everything else. I've had some of those type of disk's like a brilo pad and there fantastic for getting the paint off.
I've had trouble with flap discs for removing rust. I have 40 grit, but they do tend to get clogged by the rust. Strip and clean discs are effective but are worn away quickly. So I use grinding discs for the worst of the rust then the others to tidy up afterwards.
if there is any pitting at all then flap discs won't do the job (as to completely get the pits out leaves the metal too thin), the strip and clean discs are far more effective, but as said, they do not last that long.
The other possibility is a chemical clean, Frost and others sell such chemicals. Have a look at this link- http://www.bilthamber.com/deoxgelbuy.php.
I must say I haven't yet tried them, but I intend to in the near future. Has anyone used this type of stuff?
I have used the DeoxC great stuff the same active ingredients but just supplied as a powder to mix with water, but being a professional cheapskate I also mix it with wall paper paste to make a gel, then cling film it and leave over night. Works a treat, and much cheaper than buying their gel version, also more versatile.
It also works to wrap the item in kitchen towels and soak it in the stuff.
Unusual to find a rust remover that works just like it says on the packet.
Chunko'.