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I am a small business that manufactures an automotive gauge. I need to paint letters by hand, and I'm wondering if someone could suggest a paint and/or combination of tools that would allow me to paint these letters quickly and cleanly?

The letters are machined (relief) into ABS plastic. They are 0.020" deep, and the grooves that form them are about 0.025" wide. The letters themselves are about 1/8" wide each.

I was hoping I could find a paint that I could inject into the letter using a syringe and a needle that is smaller than the groove width. But this requires a paint with low surface tension so it will fill up the letter instead of just forming a blob and not dispersing.

Attached is a picture of the letters. I have attempted to inject water based acrylic latex into them but it doesnt disperse well so it overflows.

Also the paint needs to be long lasting, UV resistant, and generally high quality because it will need to look good for years installed in an automobile cabin.

thanks for any help!

I would use enamel paint. This uses alcohol as a base so has a lower surface tension. It is also very tough.
Depending on colour, it should be pretty UV resistant - steer clear of red as in most paint types it is the least UV resistant.
You will also need to buy some thinners like white spirit both the thin the paint & wash your equipment.
Good luck.

I also think enamels are what you're after. Actually, I've seen "enameling" paints sold in a fine-tipped squeeze bottle for just this sort of application. Well, they're intended for jewelry work (replicating the look of vitreous enameling on metal cloisonne work), but the substrate doesn't matter - they're meant to fill small depressions by being injected directly out of the bottle and air-dry, instead of needing firing.

Never used them, myself, nor do I remember the brand to look up product specs, so I couldn't say if they'd be cost-effective and sufficiently resilient. I would think they're an option worth researching, at least, as it saves you the time and trouble of thinning and cleaning syringes. If they don't pan out or are too troublesome to find, bubber's suggestion sounds best.

since you are painting the exact same thing over and over again I would suggest making a jig that you could place the guage pod into with a stencil above it that has the letters open, I would then use an air brush or a "silk screen" type effect. Hand painting those would be far too time consuming imo.