I use a Micron pen. Rescued a 210/5.6 Fujinon W whose cells I bought for very very little because it had been dropped on rocks and the front element had scratches and gouges.
Yes, an old Dagor with a 9mm gouge in the center of the rear glass, it worked just fine.
A paint stick would be my last choice, as it needs to be rubbed in then the surrounding mess cleaned away. Sharpies aren't opaque. I used Speedball black lettering ink, a pigeon's quill made into a pen, and a loupe. There are fineline pigment pens available that should work.