I bought a set of these for the SLK, so far they have not rusted (in 2 years and still counting!).
These bolts have the correct 10.9 grade tensile strength required for use as wheel bolts. It has a ma tt silver appearance which is super tough and will not chip, scratch or rust, like cheap wheel bolts.
www.ebay.co.uk
Another way was recommended years ago by Television (RIP), namely to wire brush the visible bits, put the bolts thread down through a box so just the visible bits are visible, then spray with correct colour Hammerite rattle can.