Have only used the ones on the 2 bay cement hoppers, they are likely the same metal but I didn't prime them. After probably 10 years I believe they are still ok, Larry. The best color would be to match the body color, if they are visible at all they got painted the same color as everything else.
And like most Roundhouse metal castings, you'll need some metal files and lots of patience.....I haven't seen one yet that didn't need a bit of coaxing to fit.