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I must admit masonite makes me nervous, but I still use it occasionally as well. I'm surprised that sanding the masonite first doesn't cause you more problems. I had someone help me cut a bunch of little boards from an 4' by 8' tempered sheet from the lumber yard once. The saw blade we used was not fine enough or something, and we ripped up some of the edges. I just sanded those down a little, and they looked fine...until I gessoed them. Then these brown stains leaked up through the gesso everywhere I'd had a rough edge. Multiple coats did not help.
When I gesso, I never sand the masonite. I just use a paint roller or sometimes a really fine sponge to apply gesso. That usually provides plenty of texture for me. I still have to apply two or three coats to really cover up the dark surface.
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