As far as I know fishing line is made by extrusion, or pouring the liquid plastic through very small holes, and this makes microscopic jagged ridges on it which can slowly abrade whatever the object is that it's touching. Probably not a problem with, say, a rock, but it could be a problem long-term with something soft like old plastic or leather. Also the plastic line can degrade over time. So, yeah I think Jerry's right about the cotton thread. Or leather thread if there is such a thing and it's small enough to be not very visible in the finished piece. The unbuffered board would be better because it's leather and organic items (wool, silk, feathers, leather etc) like to be in an environment with a lower pH (most other boards have calcium carbonate added to get the pH up around 8.9-ish). The problem with using unbuffered boards is that your color choice is very limited... Hope this helps.