A couple hours with a random orbit sander and 100-120 grit disks will remove all the varnish or paint from a typical canoe or kayak. Buy a box of disks, because they will plug pretty quickly and they don't cut well when plugged-up. Some marine chemical strippers are specifically made to use on fiberglass and some (like Citristrip) don't seem to soften it much and can be used, though sanding is usually faster. Varnish sanding dust and resin sanding dust (as well as the sanded surfaces) are often slightly different colors, so you may be able to tell when you have removed the varnish and should stop sanding just by eye. Obviously, if you start to see the weave pattern, you have gone a bit deeper than desired and should stop sanding.
When you think you're done, wipe it down with water and look for blotchy spots where the color changes. Even with polyurethane, spots where there is still old varnish are likely to be a bit more yellow than the bare resin will be. What you see when it's wet is pretty much what you'll get in terms of color and evenness when varnished.
Point of order: There ain't no such thing as "poly". It's a meaningless term without its tail end. Also, there are a multitude of grades of polyurethane and some of them are excellent. Like any paint or varnish, the price of a can depends on how much of the content is expensive components (filters, pigments, oils, solids, etc.) and how much is cheap thinner. Some of the toughest and longest lasting clear finishes on earth are polyurethanes. You get what you pay for - surprise, surprise. Spar varnish will also work just fine on a stripper if you would rather use it. Be sure to get one that has a good UV filter in it, because its main job on a strip canoe is to protect the resin from UV damage.
