Thank you!

Both of us just used fabric paint on our zentai masks. I believe she just painted hers on, and I masked off the eyes and then painted.
There are tiny holes in the mask so that she can see through the paint, but for Slick my eyes are right above the masks' eyes since Slick/Jack has such a large forehead.
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This is the paint that I used, and it works very well, but I would do it in a couple thin coats, spread with your finger, in order to get a smooth coverage
Everyone that I've seen, that looks decent at least, has used a fabric paint for the eyes
For the eyes, the tiny holes are really small. Put the mask on and mark where your pupils are, and cut a tiny tiny hole and see how well that lets you see.


For the mouth for Slick, I actually cut a hole in my mask and made my own fake teeth for him because I wanted to be able open and close my mouth/stick my tongue out like he does. Though, you can do the same thing as the eyes and paint the mouth on. I would recommend for Slick/Jack that you do his mouth open/teeth bared. Depending on the quality of the costume, it'll help get across that it's Slick and not Droog.
If you're going to cut holes, for the mouth or eyes, make sure you cut them smaller than what you think you need, because remember that the fabric likely stretches, so it may seem larger when you have the mask on. THEN if you need it bigger, you'll have a better reference for how much you should cut