Knots for fluorocarbon. I have found the grinner (or uni) knot the best (reliable)for tying-on flies etc. For droppers we have settled-on the 'Pape' double-uni knot, with a tuck to make the thicker tag-end stick-out to tye-on the dropper fly, sorry for the complicated explanation! believe me, once shown this knot you will not use all the other old fashioned knots, designed for nylon line.
My pals and I have spent ages of our 'sad' lives stood in my tackle-shop playing with all different brands of fluor line and the double-uni wins every time with all brands, suffice to say though that some brands perform better than others, the dearest brands do not perform as well as other more affordable ones, (paying for the packaging and name)
A rep once told me that the very smart box his brand of fluor line comes in, costs more than the actual line and spool in the box!!! enough said.