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  1. #1
    Join Date
    Sep 2008
    Posts
    3

    Default Joint compound skim coat over plaster is now cracking.

    I had old plaster walls, the previous owner had done a horrible rough skim coat of joint compound over them.

    I dug out all the cracks, scrapped the rough joint compound and skim coated over the walls with new premixed all purpose joint compound. The paint, if still on was a flat surface and I thought I wouldnt have any problem with bonding.

    Now, there are many hairline cracks running through the surface, I can push on the surface and get some movement. I scrapped off some areas and the new coat didnt seems to bond that well with the old. Other areas the old joint compound is cracking as well.

    Should I re-sc****, prime and reapply? Did I use the wrong materials? What is the proper way to do this, I have other rooms needing help.

    It was sooo much work! and now I am really dissappointed.

    Any help is great

  2. #2
    Join Date
    Mar 2009
    Posts
    1

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

    We had a decent amount of cracks in our plaster around doorways and windows, some were pretty deep. It was suggested to us that we use paintable silicone. I scrapped the cracks and excess plaster, applied the silicone to the crack, and then smoothed it out with a puddy knife. Once the silicone dried we painted over it. After two coats of paint, you can't even see where we applied the silicone.

    Overall we've been happy with the results. A couple of the cracks have reformed slightly, but that can be patched over again with silicone. The reason we used silicone is that it will expand and contract with the walls & changing temperatures throughout the year.

    I'm not sure this will entirely answer your question, I wasn't sure if you were redoing the entire wall or just tackling a couple of cracks here and there.

  3. #3
    Join Date
    Apr 2009
    Posts
    1

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

    DJSully- I'm experiencing the same exact problems. Did you find a solution? I'm desperate here!

    Thanks

  4. #4
    Join Date
    May 2008
    Location
    Denver and Dublin
    Posts
    63

    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!

  5. #5
    Join Date
    Nov 2007
    Location
    Sand Springs, OK
    Posts
    467

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

    I think the old owner missed a vital step. He forgot to secure the cracks before he applied filler and skimmed over it. After all you need to anchor all the ingoing "filler into the lath as well and not depend on the plaster grabbing onto it and holding it forever.

    Follow these steps and use these products and you can't go wrong.
    Ask This Old House How To
    Debby in Oklahoma

  6. #6
    Join Date
    Aug 2009
    Posts
    22

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

    At least you can easily sand it off.

    Use mixing compound (never premixed). ready mix air dries (if it chemically dried, you'd have a solid block when you opened the can). This doesn't go well with a plaster wall and isn't made for wide, thin surfaces that don't allow it to grab.

    I'd try using a mixing compound that you mix up and apply, you can get it thin and it will sand just like joint compound. The alternative is to replaster a skim coat with real plaster.

    A lot of "experts" are stating that you should use joint compound for everything, but it just isn't made for plaster.

    you may also want to coat the wall with PVA adhesive before you start coating, which will help the top coat adhere to the slick plaster wall.

  7. #7
    Join Date
    Aug 2009
    Location
    Ladson,SC
    Posts
    192

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

    Quote Originally Posted by djsully View Post
    I had old plaster walls, the previous owner had done a horrible rough skim coat of joint compound over them.

    I dug out all the cracks, scrapped the rough joint compound and skim coated over the walls with new premixed all purpose joint compound. The paint, if still on was a flat surface and I thought I wouldnt have any problem with bonding.

    Now, there are many hairline cracks running through the surface, I can push on the surface and get some movement. I scrapped off some areas and the new coat didnt seems to bond that well with the old. Other areas the old joint compound is cracking as well.

    Should I re-sc****, prime and reapply? Did I use the wrong materials? What is the proper way to do this, I have other rooms needing help.

    It was sooo much work! and now I am really dissappointed.

    Any help is great
    The use of any type of DRYWALL COMPOUND OVER PLASTER WILL FAIL.
    If it is real plaster than use a plaster product to do all repairs.
    If you are trying to put a plaster product over a painted surface make sure that it states that it can be applied over paint if it is raw plaster any plaster product will be ok.
    Chech out the sites for Venetian plasterand look for info that states ALL NATURAL LIME.
    DO NOT USE SYNTHETIC OR ACRYLIC BASED PLASTER PRODUCTS most of these are a paint product.
    I have used American Plaster (this is not for wet areas)
    Franklin Stucco (works very well may need a primer if not used over raw plaster surface)
    Firenze Enterprises Inc.(have not used but it is lime based.
    Master of Plaster ( used a lot of this product no problems to date NO bonding agent reqiured)
    ALL OF THE ABOVE ARE HIGH $$$.$$ but well worth it in my opinion most are user friendly and no sanding required.

  8. #8
    Join Date
    Feb 2008
    Posts
    21

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

    I know what you are going through. The following has worked for me. My full plaster stairwell had 3 coats of wallpaper on walls and ceiling. After scraping I found the skin coat process to be daunting with a full blown stairway height. In this area I found that instead of going through the re-caot process I used a very light textured paint (primed the walls first) and it solved the problem with a nice look. Light Texture is the key to keep it from looking like the old, tired stucco walls.

    I would consider replacing them with drywall if at all possible. I am a 55 yr. young single woman and I replaced the walls in my bathroom and kitchen, including ceiling, myself and am so glad that I did. I was able to access any wiring and other issues that needed upgrading at the time and eliminate future problems that would mean breaking into the plaster later anyways. I did not find tearing out the plaster as bad as some said and in the long run I think that the time it would take to try to get the right product and coat old plaster was not worth the effort and I ended up with outstanding new walls. I left the lathe on though and may have re-considered that if I did it again. It was also easier to add trims and any shelving, etc. that would have been a bear with the plaster. A great time to insulate too and you now have new walls, good electrical and added insulation, if needed. And really, not a lot more work for the results. Just a thought.

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