A question of chatzitza

I've decided to postpone the series on checking tefillin for a week or so, when I get a load of new parshiyos which should provide some material. In the meantime, here is a halachic question for my vast readership. Come on, at least two of you are rabbis. :)

There are various foul-smelling chemicals available to sofrim to clean klaf, smooth it, or produce other desired effects. Every once in a while, a new additive comes out and people wonder if it constitutes a chatzitza. The latest round of this debate is about a product which raises a variety of other questions, but one remark that I read piqued my interest: "A layer one millimeter thick is obviously a chatzitza and one molecule is obviously not, but what is the cut-off?"

1. Is that premise true? Or is a contiguous layer deliberately applied to the klaf (or the hand, or anything else where these laws come into play) a chatzitza regardless of its thickness? Do those criteria even matter?
2. Even granting the premise, is that discussed anywhere? 

The Rambam in Yesodei Torah 6 about erasing Shemot on your skin comes to mind but it's unhelpful because all it tells us is that there is something that can block the water without being a halachic barrier. Same for the halacha about ink on one's hands for נטילת ידים -- the fact that you can wash means the ink is too thin to constitute a chatzitza, but I'm not sure there is any upper limit anywhere.

House keeping

Wise words from Randall Munroe of xkcd.com

Since I only have about five readers anyway, that part is taken care of. Content generation will start on Thursday afternoon with a series on checking tefillin.