Just a couple of comments on Joaos post......
.... the only time I've met a calliper that was beyond repair was one that had a cross-threaded (or incorrect thread) bleed nipple rusted solid - otherwise, they rarely get 'too' bad to refurb.
.... Phosphoric acid is marvellous stuff (and I've been banging on about it for years, so I'm surprised it took Joao this long to believe me
) - it can be purchased online at loads of places, and can often be got from a 'local' pool/spa/hot-tub/aquarium/hydroponics type place where it is sold as "Ph down" or similar, where it is generally 81% purity.
Although any concentrated chemical can be dangerous, this stuff is actually quite mild (compared to many) as it is used in the food industry... which is why Coca Cola works well as a rust remover. I would always recommend using rubber gloves, and keeping it in a plastic container. If diluted to 10% it is great for soaking rusted parts, or just paint it on - rinse well with hot water when done, and you'll see when it's done as the brown rust will change to black Iron Phosphate, and the 'clean' steel will be matt grey in colour. It will take paint without any special treatment after that.
At has a bit of a habit of dissolving some metals, so take care if using it on zinc-coated (galvanised) bolts etc, and any chromework..... basically, if it fizzes, the metal is dissolving!
I only caustion against using anything too abrasive on the piston as and deep scratches can give a pathway for fluid to get past the rubber seal and leak..... but as long as there is no damage, there is no problem (but I'd still just chuck a new piston in anyway!).