For touch ups I would suggest, with the right ink, the technical pen would be far superior to a sharpie because you can just set the pen tip down and ink will flow. No need to rub it around. I always found sharpies difficult to get a good / durable layer built up. The tech pen ink used to be much more resistant to etch as well. I see the bottle I have is Staedtler #485 23SAR-9. The R may be a P. There's a bit of ink dried on the container. A google search on anything to do with technical pens draws a blank as far as I can see. Staedtler does still have a selection of "sharpie" type markers that may be better than an actual Sharpie.
I believe they also had a technical pen ink that was used somehow in the ceramics industry. Maybe to autograph mugs or something?
That may be another place to look for a good ink.
Sage