Canvas gazebos are the tent-like movable structures sold in most major hardware store’s landscaping departments. The problem with canvas gazebos, even if they come from the manufacturer coated with waterproofing, is that the canvas usually does not resist water for more than a season. The easiest way to add a new coat of water-proofing to a gazebo of this type, if you do not own an extremely tall ladder, is remove the canvas before treatment.
Unhook the snaps and Velcro loops that attach the canvas to the metal frame. Slide the canvas up and over from one side, until it slides all the way off of the frame on the opposite side.
Inspect the metal frame for signs of rust. If any rust is found, sand the area, and spray on a coat of rust inhibiting paint by holding the can upright and pressing the nozzle, 8 inches away from the frame. Allow the frame to dry completely while you waterproof the canvas part of the gazebo.
If the ground is muddy, spread the tarp out on the ground and then spread the gazebo canvas on top. If the ground is dry you can spread the canvas part of the gazebo out flat on the ground. If the canvas is wide enough that you can't easily reach the center, roll the canvas in from the edges until you can reach the center.
Shake the can of waterproof spray, hold the can at a 45 degree angle with the nozzle 8 inches above the fabric. Press the nozzle, and apply the waterproof spray by sweeping the can from side to side in a wide, smooth, and continuous motion. If you rolled up the edges, unroll them when you can't spray any further out. Spray the entire canvas surface with a heavy coat of waterproofing.
Add a second coat of waterproof spray, and then reattach the canvas to the frame by working in the opposite order of the way you took the canvas off.