Black Phosphate Treatment?

Phosphate is an easy DIY, just read some articles and ensure you run a few fistful of nails or steel wool in the bath before parts.

Also, since it's an acid process, you need to do a hydrogen embrittlement relief by baking the parts at about 350F for an hour or two. Otherwise you can sometimes get brittle fractures. Uncommon, but it's so cheap and easy to do the relief that you might as well.

Lps3 is a great long term rust inhibitor and can work well on underbody hardware.