Hi Jennifer,
I reviewed your question about Masonite. I am not a chemist or whatever it takes to be positive but I have used Masonite for years now (about 20). Until recently I used only untempered but agree with Jim on this one. I read that the tempered is now fine to paint upon because of the way they temper it and lack of the usualy oils.
As for technique to get it ready to paint upon it is a follows: Lightly sand with sandpaper first to develope tooth (place for the Gesso to grab hold). I then use Acrylic Gesso and I use a foam brush (found by the paint brushes in all paint departments) to apply a thin coat. I do this quickly and in one direction only, smoothing as I go. This will dry within minutes (approx. 15) depending on how thick you apply. I then sand with a very fine sandpaper (400 to 600 grit) I find it mostly in lumber places or where they sell car parts as it is used in finishing car painting or detailing).
I spritz or lightly spray water on it (very lightly as not to take off the gesso) and wet-sand with this 600 grit. I then coat with gesso again. Then sand again. I will do this at least 3 times and sometimes up to 7 or 8.
It will then be as slick as glass. I love this painting surface as I paint in great detail. I also would like to add this. Jim said that a lumber yard will cut it. In my findings--be very careful here. I am fortunate that my hubby cuts mine and has all the fine tooth saws and he is very careful to "square" the pieces perfectly as they need to fit exactly into your frame. Most lumber yards are hurried and cut to fit walls , etc. and first, are unaware that they need to perfectly square the pieces. You might try telling them of the perfection thing. If you have a friend or possibly cabinet makers or carpenters who posess a good table saw----this is your best bet.
Good luck and although this is lengthy, I hope this has given you an idea.
P. S. I also re-gesso some canvases with same technique to get a very fine tooth. Although, linen portrait canvases are great too.
Happy gessoing,
Patt