I purchased a set of these accessories for my SLK. To far, they have shown no signs of rust, even after a period of two years and ongoing.
Another method, which was suggested years ago by Television (Rest in Peace), involves using a wire brush to clean the visible parts, inserting the bolts through a box with the threads facing downwards so that just the visible parts are exposed, and then applying a coat of the appropriate colour Hammerite paint using a spray can.
https://www.ebay.co.uk/itm/362248505646?mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338181125&toolid=10001&customid=2518X1570512Xb19c944e7a7f8169c3c708352b24e28a