Hi,
I've just redone the oil level indicator plates on an old mill. I used a technique I learned from my dad some 50 years ago.
Thin some enamal paint to about spraying consistency and using a bit of wadded rag dip in pait and then wipe across lines/marks ect in one direction only.
Useing another,(tightly), wadded clean rag wipe across surface quickly and lighty, again in one,(the same as above), direction only.
The first wipe deposits paint in the mark and the second removes surplus from the surrounding surface.
This technique is used in 'antiqueing' cheap pot-metal parts and the like.
If you don't like the result just wipe off the still freash paint and try again.
I'd post a pick but I just put them back on last night.