In my experience it makes the nylon translucent. As a rule when I am building rods, I break out a dowel and do a couple of wraps when I am not too sure about a technique. I have one with about 20 different, really crappy, wraps on it. 🙂
I’ve been using the Flex Coat color preserver and I find it okay. The technique I use is to thin the preserver 50/50 with distilled water for the first two coats. This allows the preserver to penetrate the thread better (both nylon and silk). The last coat I put on full strength. I haven’t had any trouble getting smooth coats using this technique.
I think I read about the technique in a paper from Goldenwitch about doing feather inlays. Seems to work good.
Link to Feather Inlays article