I use a fountain pen for all of my writing. I purchased MicroGlaze to use to cover dollar amounts and the identity of "to the order of" on checks to prevent alteration and smearing and to cover addressee's addresses on the front of mailing envelopes to prevent smearing which happens on exposure to rain. Worked great, no smearing even if you drop water on it. Have already recommended it to my local pen store, they were unaware of this product or anything like it.
I just learned how to use this stuff at the Great Lakes Mega Meet. It seems expensive, but you use so little it'll last a long time. Any part of a surface that you cover with this will remain the color you start with. You can then stamp, color wash, alcohol ink around the area to give depth and added color to the uncovered parts.
I really like this product, except it was a little difficult to apply over chalk without smearing. I need a better applicator than my fingertip, but otherwise, I really do like this product. It only takes a thin coat, and it protects very well.