Best way to waterproof canvas tents

Q. I am a Scout from New Zealand, and we struggle with keeping our Scout group’s canvas tents waterproof — they are pretty old. How can we make them waterproof?
— Elliot the Kiwi, Palmerston, North New Zealand

A. Good question, Elliot. And you probably win the award for the most far-flung Gear Guy questioner ever. New Zealand? Awesome! O.K., waterproofing a canvas tent. There are several waterproofing products you can use, but one of the best is Canvak ($35 USD for 1 gallon; cabelas.com). Buy a couple gallons of that stuff, and use a pump sprayer (like what you might use to treat weeds or an old Windex bottle) and spray the Canvak all over the outside of your tents. Beware: the smell is pretty strong, so you’ll want to have several days to let your tents air out before sleeping inside them.