If painting silver.... another vote for, (smooth), hammerite. Wire brush & brake cleaner 1st. I am sure there may be superior products but hammerite is cost effective. It can be white spirit thinned a little which will make application easier & drying quicker.
POR-15 a top quality product for chassis work....no idea about success of this on calipers.
Regardless of paint type I would make sure its applied thinly....to avoid deep brush marks, runs or excess paint that compromises the finish.....Some prefer to spray for a better finish but more time consuming & more masking required. Touch in's, freshening up or repainting easier by brush too.