I'm sure all you smart folks out there already know this trick, but it's new to me.
I've had terrible luck being able to mark sil with anything but the paint pens. Well, my paint pen died last night and I had some work to finish. I remembered a white-out pen I had in my desk and grabbed it. It works great and is cheaper than the paint pens (cause I get them free from work )! Bright white and easy to see; it sticks well enough that it doesn't get rubbed off as I'm moving material around the machine and it comes off with very light scratching of the fingernail.
Anyhow, maybe I helped somebody just starting to work with sil, or maybe not. Either way I'm happy.