Masonite has been used as a substrate for painting for quite some time. Its smooth surface and relative rigidity appeals to many artists who want a smooth, seamless painting, Masonite is a great choice. Although there are pre-prepared Masonite boards available, you can save some money and come with an even better product by doing it yourself. Although this seems like a daunting and monotonous process, the dividends will show in your final paintings.
Find a piece of Masonite that's the size you want, or cut one to size. If you've purchased the Masonite from an art supply store, it's probably already cut to size, but if you're getting it from a hardware store, or it's something you've found, you will have to cut it or recut it to make sure the corners are square. You can have a hardware store employee cut it for you for little to no cost.
Wipe the Masonite with a cloth dampened with mineral spirits to get rid of any unwanted residue. Depending on where you acquired your Masonite, it may have a sawdust and grit on it. Even if you can't see anything on the surface of the Masonite, it can have small particles which can wreak havoc on the rest of this process. Make sure the Masonite is completely dry before proceeding.
Using a trowel or a thick brush, apply a thin layer of gesso to the Masonite. Make this layer as thin as possible without being able to see the board underneath. Apply the layer as smoothly as possible, avoiding any streaks or brush marks. Once this layer has dried, repeat this step.
Use a heavy grit sandpaper to sand the Masonite. Your goal here is to get rid of any bumps or streaks in the gesso. After you've sanded, wipe the Masonite down with a dry cloth to get rid of as many of the sand particles as you can.
Add a third layer of gesso. Once again, make sure your application is smooth and free of bumps or streaks. You can add more layers of gesso, if desired.
Sand down the Masonite again with a heavy grit sand paper. Continue sanding the Masonite, going to a finer grit sand paper as you go. The last sandpaper pass should be done with a very fine grit sandpaper. Once you've done your last pass, wipe the board down with a dry cloth once again to remove and particles left from sanding.
Apply bits of gesso to the board at this point to provide texture to your final piece. You can splatter gesso on to create a stucco-type look, or if you already know what your composition will be, you can create a texture within certain parts of your painting. This step is optional, as you could begin painting on the untextured board.
The dust particles left from sanding gesso are very fine and they get everywhere. Make sure your in a well ventilated area when performing these steps.