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Old 06-07-2009, 10:24 PM   #1
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Right materials to patch plaster on masonry?


I'm trying to figure out what materials to use to repair a plaster wall in a bedroom in our 81 year old house. Some old water damage has worn away the brown and scratch coat underneath the plaster in one area, and the skim coat has cracked and fallen away. So we have a 2' x 2' area where the surface ranges from still having a scratch coat to just having the brown coat to no plaster at all. The plaster is applied directly to a solid masonry (brick) exterior wall of our house -- no wood or metal lathe.

I can't figure out what materials to use to make this repair, and what step by step approach to take, I've read about bonding agents, bonding plaster, base coat, finishing plaster, paster of Paris, patching plaster, and even more.

Can you recommend the right materials to use? Thanks so much!

Jim

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Old 06-08-2009, 03:51 AM   #2
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Right materials to patch plaster on masonry?


I'm not an expert, but have done a lot of repair work over the years. Two methods have worked out well for me. Wet the wall down (moist not dripping) apply a coat of one step. I usually choose a type with a low minimum application thickness for patches, name brand, they have two main types here (Knauf Redband and Goldband) with slightly different minimum and maximum recommended thicknesses and slightly different characteristics (the coarser, Goldband, is easier to work with). You can fudge the maximum recommended thickness a bit if necessary, it just gets harder to work with. Lightly spray the finshed Patch/wall with a pump sprayer (old Windex bottle) a couple of hours after your finished and then again every hour or so, so it sets slowly.
Think big, the entire patch as a unit, find something that reaches past the edges, use this to spread your plaster after you've found some way to get it on the wall . Trowel (smooth) as little as possible after it is level. Often the more you trowel, the more uneven it becomes. Don't even try until it has started to set a little.
Second method, I most always use a (spray on) deep sealer anymore (let it dry overnight), for the simple reason it slows down moisture transfer between the existing wall and your repair and can help rebond the edges (capillary action). And use a brush on bonding agent, if the existing plaster still seems iffy.
Preparation is usually to lightly tap the borders of the area to be patched, with a straight claw, body or roofing hammer, to knock off the loose stuff. The wedge or pointy end not the flat end you nail with. This is where the fun starts, you don't really know how far the damage has spread. A 2'X2' hole can turn into an 8'X8' hole, pretty quick. I use a wire brush to knock off the loose material after the hammer or a very stiff nylon scrub brush. Get the dust off with a vacuum and damp rag.
A brown coat or a bond coat, then a skim coat, may last longer or the rest of the wall may develop trouble first. In my experience the one coat outlasts the existing plaster drastically.
A good solution, other than taking the whole wall down to stone, is a sturdy wall paper. They make stuff that is basically, fiber glass and nearly indestructible (works like drywall tape, just in a big way). The wall becomes a unit. Or fur the wall and put up Sheetrock. The fur and Sheetrock, along with a layer of insulation reduces heating costs and is a do it once solution.
The main trick with plaster is the proper working consistency. Think Ready Whip, not cement. Pre mix may be the way to go for a small repair.
We have a lot of trouble here with the brown coat and the skim coat separating, they say it's because of mold (damp climate) and/or improper application. Brown coat takes a long time to dry, bonding plaster is quicker (hours), one step a couple to a few days. Drywall mud works well to fix the small mistakes or feather the edges of your patch. The drywall mud I use has a fairly high fiber content, likely some cellulose and sticks like glue.
Like I've said, I'm not an expert, but some/most of my patches are 25 years old and still there. Sometimes it is hard to tell just how extensive the problem is, a repair may not last, the surrounding plaster may let loose. If in doubt, furring and Sheetrock is likely the easiest and longest lasting solution.

Sorry I wrote a book, had to explain in a blurb.

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Old 06-08-2009, 09:18 AM   #3
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Right materials to patch plaster on masonry?


Thanks for the recommendations. This is very helpful.
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Old 06-08-2009, 12:23 PM   #4
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Right materials to patch plaster on masonry?


I've been researching skim coat to brown coat failures and most seem to be due to not enough curing time for the brown coat (it can take a month).
Sealer and primer are sometimes used interchangeably. My criteria is it has to be thin enough to spray and soak in. I guess a better description would be penetrating base coat, rather than sealer, primer or primer/sealer. Read the destructions I tend towards Knauf products (mainly insulation in the US, but they make most everything plaster related in the rest of the world) or Henkel (same people who make Loc Tite) their products generally perform as advertised.

Last edited by Bigfoot; 06-08-2009 at 12:37 PM.
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