Quoted from Rdoyle1978:

I use these:
https://www.pinrestore.com/PaintPens.html
Make sure you match the finish (matte vs gloss)

Quoted from Pinwow:

Have you ever gotten a playfield clearcoated after using them? Think it’ll clear fine? I have some very minor touchups I have to do on a playfield that I want to send and get automotive cleared

Quoted from Rdoyle1978:

I usually use them for cabinet touch ups; I don’t trust myself to do playfield touchups! I imagine they would be fine, but it depends on the type of clear used, and how well the playfield was prepared in order for the paint to adhere

Quoted from Pinwow:

Oh cool, are you using them to touch up over painted cabinets or decal’d cabinets, or both? Do you just clean the cabinet and then use the paint markers? Have to seal the paint on somehow? Thanks

Quoted from Rdoyle1978:

Depends - if it's just a quick touch-up, I'll run it over the cabinet and pass a heat gun over it REALLY briefly to set the paint. This also helps get the matte finish needed. The fat tip paint pens aren't great for detailed touch-ups, but the black works great for most nondescript cabinet areas.
Yes, ALWAYS clean the cabinets. Use a tiny amount of the Green cleaner, or something non-toxic and non-agressive/abrasive. Most of the junk on the cabinets will come right off - but you want to remove smoke stains, oils, etc. as well.
I've not had to touch up any decaled cabinets - it's always been old WPC games so they are screen printed. (not sure if you meant games with restoration decals, or maybe vs. older games which have stenciled cabinets)
The heat gun just helps dry it and seal it. If you really want to get serious, you can get paint that is specifically made by design to be sealed by heat. That stuff almost never comes off, but it's more expensive and usually not necessary unless you're working on a playfield.