On a guided tour of a Bentley dealer, the service manager took great pride in emphasising their attention to detail by showing me one of the mechanics painting the brake calipers on a modern Bentley that was in for a service. I've been doing this for years on our fleet of classic cars for a number of reasons.
Anyone who has bought a classic car that hasn't had much use knows that rust on the brake calipers can cause problems. It is unsightly when viewed through nice gleaming chrome wire wheels or period alloy wheels. But most importantly rust can actually cause problems with the braking system when servicing.
The calipers are normally made out of cast iron and while some cars have nickel plated or, better still, copper brake pipes, virtually all of them have steel fittings on the brake pipes and steel brake bleed nipples which can rust. If the rust isn't kept under control then the bleed nipples can rust into the calipers making it virtually impossible to bleed the brakes. I have seen too many cars where the flats on the bleed nipples have been rounded off so that they can no longer be undone with a spanner. In severe cases I have known of bleed nipples that have snapped off, meaning the caliper has to be removed, brake pipes removed (if they will come undone, and then the bleed nipple very carefully drilled out and replaced.
If the bleed nipple has reached this stage then it is quite possible that the brake pistons have started to rust in the calipers as well - but that is a separate issue.
When I do my main winter service on all my classic cars I remove any rust from the calipers with a wire brush, particularly around the brake pipe ends and the bleed nipples. I treat these items with a rust preventer, normally Kurust, and I then give the calipers a thin coat of Smoothrite paint, normally in silver. Some brake calipers were cadmium plated when new (which has now been banned) which has a yellow / gold colour. In this case I use the gold Smoothrite which dries to a similar colour to cadmium.
I emphasise here a 'thin' coat of paint. Just enough to give a good covering and brushed out thinly. DO NOT paint the brake pipe ends or the bleed nipples or the part of the caliper that the nipple screws into, otherwise this can seize up with paint which is as bad as it rusting in situ. If you remember, it is also worthwhile adding one drop of thin oil or a squirt of WD40 (or equivalent) to the base of the nipple to allow it to seep into the top couple of threads.
Some of our cars have drum brakes on the rear, so here I remove the drums, de-rust, Kurust and paint them, normally in silver Smoothrite. The brake pipes and bleed nipples don't fit into the drums but through the drum back plate, so while the wheel and drums are removed, I wire brush the pipe ending and nipples and Kurust these as well.
You now have a set of rust free, very tidy looking, calipers and drums with fittings that hopefully will undo when needed. Nice to know that Bentley in Cheltenham come up to my standards.
At this point it is also worth adding a reminder that brake fluid absorbs moisture from the air and therefore should be flushed out and changed every three years, hence the need to be able to undo the bleed nipples.