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  1. #4
    Join Date
    May 2008
    Denver and Dublin

    Default Re: Joint compound skim coat over plaster is now cracking.

    All purpose joint compound, as the name may imply, is for joints. And it's certainly not “all-purpose”. Its used for joints in drywall where an appropriate paper or fiber backing has been applied.

    If you're resurfacing an area, you'd want to use a topping compound which is entirely different thing. You won't want to use either over plaster. The old lathe and plaster is a lime based mortar and it's, basically, vertical concrete. The only thing that keeps it from cracking is that it's very hard; if the wall moves it moves. If it's cracking, it, and anything that covers it, is going to crack.

    You cannot apply either joint compound or topping mix directly to a plaster wall reliably because you'd be applying it to paint applied to paint applied to, etc., applied to plaster and it won't create a reliable surface. You're applying firm calcium plaster to latex paint to more paint to firm lime plaster to firm cement and, finally, to a moving structure. It doesn't work if the structure moves.

    Joint and topping mix have no latex in them and they don't move. If the wall moves in any direction, they crack. You could try mixing your own topping compound from a drywall powder mix and using a latex instead of water.

    You can stop the cracks by removing the lathe and plaster and rebuilding the wall surface with more modern materials. Unfortunately, that's the only thing that works with any reliability.
    Last edited by KKelly; 04-19-2009 at 02:52 PM.
    If I only knew what I was doing!

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