Example; 400/4 brake caliper body
After removing the caliper from the bike and draining the old fluid (taking care to dispose of it in a safe way and keeping it well away from any painted surfaces) you will probably find that it has at least some of the following problems;
Dealing with the first point, I have found that by far the best way to remove any seized piston from a caliper, is not to try to use the effort from the master cylinder, which rarely works due to inadequate pressure, but to drain and remove it, blank off the fluid inlet with a suitable short bolt with the correct thread. This must be at least the equivalent of a full nut in depth but must not come into contact with the machined cone at the bottom of the hole and sealed with a copper or aluminium, sealing washer. It is then a simple matter of attaching a grease nipple to the bleed valve hole and forcing the offending stubborn piston out easily with the 5000 psi from your hand held grease gun. The added advantage over using fluid or compressed air is that the grease does not spray everywhere when the piston finally emerges from the bore.
It is worthwhile checking at this point, after cleaning the caliper, that all threads and mounting holes are in serviceable condition and free from burrs, and that the counterbore for the seal is clear of the usual fur, which develops behind it.
The best way forward now is to brush the whole job with Nitromors and leave it for a while. The original finish should quickly start to wrinkle and come away from the surface. I have found that the best way to remove the jellied mess is to use those small stainless steel pan scrubs (available from Wilkos), at the same time wearing a pair of those poncy latex gloves. You will probably have to do this more than once and scrat around in the corners with a small terminal screwdriver and a scriber, but it will come clean. You should now have a fairly shiny alloy caliper, perhaps with some previous deep scratches. The whole exterior can now be lightly wire brushed and then rubbed with 120 grit paper, to remove any deep scratches. Keep working alternately at right angles until you are happy with the surface. Rubbing with 320 grade, again alternating the direction, will now establish a surface which is good enough for paint. The big problem with aluminium is that the very tough oxides start to form very quickly if left exposed and this makes it very difficult to get paint to adhere effectively. So now you need to mask the bits you don’t want paint in or on and plug the threaded holes with whatever you can find (Plasticene or Blu-Tack is excellent for this). I have found that Upol 1K etch primer in spray cans (available from body shop suppliers) is great for the first coat and without rubbing down (unless there are monks and nuns) can be covered easily with Hycote primer surfacer. After drying (I assist this process with a hot air gun on small parts- once you have STOPPED spraying and let the air clear!!), you might want to very lightly flat the surface with 500 grade paper, avoiding rubbing through any edges (he said, glibly!). You are now ready to apply the top-coat, which in the case of Honda Calipers will be Satin Black (again from Hycote). As always, 3 or 4 light coats with a short spell in between, are preferable to one thick coat, which has a tendency to sag or run. This should now give you a very professional finish and with a little care, will have cost very little, as you still have materials to do a great many other small parts. The technique works well on a great number of those fiddly little jobs, such as clocks, footrests, stands, master cylinders (even though they were originally black anodised). This can improve the look of any machine, whether needing restoration or just a tidy up. This is by no means meant to be the definitive answer to refinishing, but merely a cost effective method, which can be employed in the home workshop at reasonable cost and with relative safety.