How to blacken ther marking on a ruler?

[6270_Productions]

Gun bluing. Apply over the whole ruler, then very lightly sand with 600 grit wet dry sandpaper. The same approach works to restore saw etchings. It won't be as dark as the original, but it should work. If the markings on the ruler were not etched to begin with, you won't be able to restore them. Some less expensive rulers have the measurements silk screened. When those wear off, they are gone.

Looks like a stainless steel ruler to me.  I'll bet five internet bucks gun bluing won't work well, if it is stainless steel.

[AHill]

Looks like a stainless steel ruler to me.  I'll bet five internet bucks gun bluing won't work well, if it is stainless steel.

Good catch, and true for most traditional bluing formulas.  Brownell's Oxynate No. 84 works on stainless, but it's quite pricey and it's a hot process, not for the typical home shop.  Probably too much to invest in restoring a ruler worth less than the bluing, though.

[Cooler]

Engravers use a paint stick:  http://www.engraverssolutions.com/PDFs/t...illing.pdf

https://www.mscdirect.com/browse/tnpla/3...gLdVPD_BwE

[teetomterrific]

This stuff should do the trick but it's not cheap. http://www.caswellplating.com/stainless-...l-kit.html