Walls – How to repair crumbling wall plaster

Run a strip of masking tape on the wall where the molding will cover it up, but where your nails will hit it.

I hammer nails into old plaster all the time by putting a piece of masking tape on the wall and hammering into it. It holds the plaster together while the nail hits it and can easily be ripped off (gently) after I'm done hitting it. In your application, you can just leave it hidden behind the boards!

Use masking tape because as, for example, duct or painters tape dry they grab onto the wall harder. I have pulled large chunks of plaster out by trying to remove old duct tape. I have successfully pulled fresh duct tape off of plaster walls, but I don't recommend trying.

I'd use veneer plaster. Your instinct about joint compound and spackling is on target, and you might get Plaster of Paris to work if you get the mix perfect - but why bother.

My guess is that the people you've been talking to at hardware stores either don't have a clue what you are looking for or want to direct you to something in stock. I'd check at a building supply store (you might get lucky at a big box store, especially if they carry blue board) that contractors in your area use.

Veneer plaster is extremely uncommon in my area, and I can get it when I need it from any of the big chain stores (although I usually have to order it - they never stock it).