I have made a rather complex papier maché sculpture which I wish to make even harder than it is, then sand and use either acrylic washes or inks on top, and then be able to use colored pencil on top of that. What products would you suggest.
There are several things you may wish to try to stiffen papier maché. First, try a 1:1 blend of GOLDEN GAC 200 and GAC 500. These are thin polymer mediums, which should penetrate and seal the paper very well. The 200 is a hard acrylic and should help to stiffen. We would suggest at least 3 even coats. We would also suggest you try adding these acrylics into the papier mache mixture with the plaster. We make an Acrylic Modifier for Plaster, but try these first as they are more readily available.

Once these layers have dried for at least 24 hours after the last application, then the Hard Molding Paste or regular Molding Paste can be applied. The hard version has a bit of self leveling to it, so if you're after a smooth finish or just mild texture, this is the one to use. Once you apply it and allow several days for it to cure, it can be sanded, painted, carved, etc.

Once you have developed this surface, you may wish to see how well it takes the colored pencil. It may be fine, but you may wish to apply a coat or two of the Acrylic Ground for Pastel, or the Absorbent Ground. Some pencil artists like the softer paper-like feel of the Absorbent Ground. We would suggest creating a test panel with it to see if it works for your application. You may find simply sanding the Molding Paste surface will be adequate as well.