A search of the forums of "stripping varnish " pulls nearly 90 posts up. Try it....;
Most guys use a methyl chloride based product. Some use the water soluable "green" products with success. I have used both. The methyl chloride worked best. I now contract with a professional furniture stripper that removes me from the smell, time, and tedium of stripping. Figuring in the time and cost of product involved, I break even or come out slightly ahead on having a commercial stripper do the work. My guy gets the canoe cleaned out to where it only needs light sanding prior to application of the first varnish coat. He charges just over $200 for a 16'er
Dave I've stripped several canoes and have tried many of the off the shelf products. Have finally found that I like and does the job with several layers of old varnish. It's Dad's Easy Spray. You can use it with the spray nozzle or lather it on with a brush. It's quite toxic from what I've read and does burn the skin so you'll need a pair of good quality rubber gloves - preferably with long cuffs. Good luck.