I know this question has been here a while, but I just found this section on Scratchboard yesterday.
I have made my own scratchboard and it is a little time consuming, but it worked very well. Here is how I did it.
On a piece of masonite start by lightly sanding the surface then paint one coat of gesso on top in ONE direction. Let that dry. Then sand lightly. Then add another coat of gesso, but apply in the opposite direction of the first and let that dry. Sand again but use a finer and finer grit as you progress. You do this layer of gesso thing 5 or six times. You want a somewhat thick but very smooth surface. After this is done and dried and sanded apply india ink in one direction only, the foam brushes work well for this (no fuzz). Let dry. Then apply another coat in the opposite direction. Do 4 layers of india ink and remember to go in the opposite direction each time.
Using this process I have a scratchboard from twenty years ago that is still in beautiful condition and the suface was very nice to work with.
Sorry it took so long to find you all!