Will plaster bond to a cement basecoat ?
I am building an ICF house (insulated concrete form). I also have some formed and poured interior concrete walls. I would like to use plaster on both the interior concrete walls and on the interior Styrofoam forms. We have coated the concrete with a cementitious stucco base coat to give it a smooth finish. We have also coated the foam with a cementitious stucco basecoat with a mesh. Can we put a thin smooth finish plaster on top of that? Red Top? Kal Kote?
Technically, plaster won't bond to a cement basecoat without painting first with a bonding agent like Plaster weld.
A heavy gauged white mortar should bond to a cement basecoat if the surface is rough and the pores are open. I saw a bunch of bathrooms done this way in a hospital with good results, and I did this many years ago in a government building with good results. However I had bad results putting white mortar, that is plaster finish, on a cement basecoat in a school. I don't want to tell you where it is, because the plaster is probably popping off the wall. I did it over 20 years ago, but still I feel bad. It seemed like the mortar never set up all the way.
If I was going to stand behind the work I would:
1. Paint the concrete, or cement basecoat, with Plaster weld. Preferably, the brand Plaster Weld, Euco weld or Tamms weld. I would stay away from other brands as they may fail.
2. Put on a veneer plaster basecoat. Without this, your finish mortar won't set up al the way. You want to spend the money for veneer plaster basecoat, and not structolite or hard wall. Veneer basecoats allow you to put on a thin basecoat with exceptional strength.
3. Use lime and gauging or lime and molding plaster heavy gauged, that is, 1 to 1 As far as the mesh with the cement basecoat, I have never plaster over this, but if it seems solid I don't see why it would fail, provided you use plaster weld and a veneer basecoat.
I have put on miles of Kal Kote and Red top finish plaster over veneer basecoat on blue board. They are pre mixed finish plasters just requiring adding water and mixing with a drill. My experience with these is they are gauged too light, that is about 3 parts lime to 1 part gauging plaster. I worked for a company almost 40 years ago that used Kal Kote finish plaster on veneer basecoat. We plastered a huge building and had problems with shrinkage. We had to go back the next day and point up cat faces, Also, the surface wasn't nearly as hard as heavy gauged gauging plaster and lime. It is my preference to gauge lime and gauging on the mortar board, and gauge the mortar heavy, that is 1 part gauging plaster (or molding plaster) to 1 part lime putty. This provides a harder surface and no shrinkage.