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There's no set amount of water that you'll need to add. Instead, scoop the primer into the bucket or cup that you'll be using to hold it. Then, add a little bit of water at a time while stirring the gesso and continue adding water until it has the consistency of a heavy cream. Your gesso will probably be really thick, so it's important to dilute it to make it easier to apply and smooth out. It's better to shoot for too little water rather than too much, since if you add too much water to your gesso, your only choice is to start this process over. Go from the top of the canvas to the bottom, stroking parallel to the length of the canvas. Cover the canvas completely while also spreading the primer as evenly and thinly as possible. Try to create as flat a surface as possible with the gesso. For best results, use a stiff brush with relatively firm fibers. If your canvas is pretty big, you can also use a roller brush instead. Be sure to paint the edges of the canvas as well; these can sometimes be hard to hit with just vertical strokes. Gesso doesn't take long to dry, but give it at least an hour to completely dry for best results. Use fine sandpaper to give your gesso a once-over and give it a smooth texture moving forward. If you don't want your canvas to have a smooth texture, you can choose to skip this part. Be sure to brush off the dust from your canvas after you sand the gesso. While the gesso is drying, take this time to clean off your brush. If you leave the gesso on the bristles for too long, it will be very difficult to remove. When you go to apply this second coat of gesso, be sure to turn the canvas 90 degrees first. This way, when you apply the primer with vertical strokes, these strokes will be perpendicular to the ones you used for the first coat. Using perpendicular strokes will make the finish of your texture much more even. You can apply as many coats of primer as you like, but most people choose to apply 2 for a smoother texture. Just 1 coat of gesso will give your canvas a relatively rougher finish. For best results, give your second coat an hour to dry before you go to sand it. Use very light pressure and short back-and-forth motions to avoid scraping off too much of the gesso. Be sure to remove the dust from your canvas after you sand it. Once you've finished sanding the gesso, your canvas is ready to be painted on!
Stir your primer and add a touch of water to it to dilute it. Use a wide brush to apply the gesso in vertical strokes. Allow this first coat to dry, then lightly sand it to smooth it out. Repeat these steps to apply a second coat using perpendicular strokes. Sand the gesso after it's dried for a smoother surface.