Hello - I am a new rod builder and want some advice on varnishing a bamboo rod.
Should I varnish before mounting the reel seat and grip? Thanks for the help.

Usually a rod is first varnished immediately after completion of the blank to seal out any moisture as you have spent a great deal of time tempering and heat treating to bake out the moisture content in the cane, so the "raw" rod blank is usually sealed with a quick application of wipe-on varnish or tru-oil to prevent the re-absobsion of moisture. Once the rod is wrapped (or whipped in the bamboo world) and several coats of varnish applied to the wraps, the finished product is then dipped into a dip tank, being very careful not to dunk the front end of the cork grasp and slowly drawn out of the tank. To do this properly, you need to stop the "pull" at each lower end of each wrap to allow the excess varnish to flow off, otherwise the excess varnish collected on the line guide will flow down later and cause a run in your finish. However, some of the masters of the "Golden Age" of cane rods actually varnished rods with a brush rather than dipping them!

You can seal the blank if you want, it's debatable about if it makes any difference, wood will stabilize with the environment even after it's varnished. Most find it easier to work with the thread if the blank is wrapped first, the wraps sealed and then the whole thing varnished afterward, usually in a dip tube, stopping at the ends of each wraps, as was said, briefly(10 seconds works for me) to let the excess run back into the tube so it doesn't cause a run. That's today's standard, but the vast majority of bamboo rods made 'back in the day" were either sprayed or brushed. Jim Payne's rods have always set the standard of excellence and they were brushed most of the mass produced rods were sprayed.
john