I have an 1890’s building with plaster walls probably from the 1920’s or before. The plaster was installed for wallpaper since it has a rather rough texture. In one room, a skim coat of finish plaster had been put over the top of this original plaster (date unknown) and has developed several cracks both along cracks in the original plaster as well as surface cracks in the finish coat. The skim coat comes off the original plaster easily with a putty knife. I am in the process of removing the surface coat back to the original plaster, but would like to paint the room again. Since the original plaster is too rough to paint, I will be skim coating the plaster again. As the previous skim coat did not form a good bond with the underlying plaster, I feel the original plaster was probably coated with a wallpaper sizing when it was originally installed. Is there anything I can do aside from scrubbing the original plaster with a strong solution of TSP to get the new skim coat to adhere to the original plaster and will the skim coat stick this time? Thanks. — Calvin Wells
I suspect you are right in assuming this rough and relatively soft plaster was intended for papering. The skim coat may have failed for many reasons, but the most likely is differential expansion. The underlying plaster and wood lath system moved with changes in moisture and possibly structural shifting. The stiffer skim coat cracked because it was stronger and then lost its bond at the relatively weak surface of the old plaster.
I suggest you back up a little and reconsider your plan to skim coat. The original builders knew this plaster would move and that is why they papered. What ever you do, it should allow for the continuing movement. If you really don’t want to repaper you could applying a fabric backing to the surface and then paint the fabric with latex paint which is more flexible than most oil-based paints.
If you must skim coat, stabilize the old plaster surface by coating with bonding agent. Bonding agent will soak into the porous surface sealing it and acting like an adhesive for the skim coat. It is available at masonry suppliers. I can’t recommend a specific skim coat material without seeing the wall, but whatever you put on should not be stronger than the plaster beneath or you will run into problems later on. Test different kinds of joint compound, fillers and skim coat plasters by applying them to cardboard sealed with bonding agent. Determine which is weakest and most flexible by bending the cardboard, breaking the coating.
If this is too much trouble, The Glidden Company makes a plaster impregnated fabric product that you hang on the wall, dampen and trowel into place. This bridges over a certain amount of gaps and cracks in the old plaster surface.
Always test your selected materials and methods of application on a small section of the wall in an out of the way place to be sure they work and give you the results you want.
Structural Assessments & Repairs
George Yonnone Restorations
Box 278. West Stockbridge MA 01266
Plaster Repairs & Materials
Here are two products of this type:
By Glidden Paints, available at ICI Dulux Paint Centers around the country.
John Leeke is a preservation consultant who helps homeowners, contractors and architects understand and maintain their historic buildings. You can contact him at 26 Higgins St., Portland, Maine, 04103; or by E-mail: johnleeke@HistoricHomeWorks.com; or log onto his website at: www.HistoricHomeWorks.com
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