I have a house that was built in 1901 and needs a lot of plaster work. I've tried both Structolite and Gypsolite as a base coat over bare brick. Both of them crack as they dry, less than an hour after the structolite starts setting They look like a dried up lake bed. I've tried wetting the substrate before applying it but it doesn't help. Could this be that it's not packed tightly enough and it shrinks when it's drying? It seems solid enough, even with the cracks but I've seen pros using it without the same problem, so I know it can be done. Unfortunately the pros didn't do a good job of spreading it flat, so I'm redoing it myself.

  • Hello, and welcome to Home Improvement. Would you add a picture of the problem? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Sep 22 '19 at 2:09
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    I wonder if you are mixing it too wet? I have no personal experience with the named product, but it's a common beginner problem with many "masonry/plaster" products to overwet them when mixing. – Ecnerwal Sep 22 '19 at 17:38

The Structolite is drying out too fast, because the bare brick is wicking out all of the moisture.

You need to use a bonding agent and possibly add some sand to your base coat, as well as scarify it, for any successive coats.

If this project is going to have any substantial thickness, it probably wouldn't hurt to also add some kind of structural support, like wood and/or metal lathe.

I use the latter all of the time, when doing repairs where the plaster has fallen completely off sections of wood lathe.

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  • I used bonding agent where my patch touched existing plaster and it turned out a lot better. I also made sure the brick was nice and wet before plastering. I mixed the structolite pretty dry, according to the comment from @Ecnerwal, which might have helped too. I noticed it cracks more easily at the edge of the patch unless the edge is against another surface. Call me a purist but I didn't want to use bonding agent directly on the brick, or any kind of fiber mesh. It's not the way they built it originally. – dwilli Oct 1 '19 at 2:23
  • @dwilli They did what they did with the technology they had at the time and most likely with some materials that are now illegal for use. Bonding agent on the brick would not violate any "purist" philosophy, if it gets the job done, while ensuring durable adhesion of the plaster. Now hanging flat Sheetrock, right smack in the middle of a 100 year old, textured, plaster wall... that's sacrilege. – tahwos Oct 6 '19 at 15:52

From my experience if there is a crack below the surface it will crack in the same place if not reenforced, I find fiber / fiberglass mesh a big help but not 100% effective. My best results come from removing loose material, roughing up the area, filling and working in fiber mesh. If there is a divot or low spot a quick pass with a grinder and a 2nd coat with another layer of mesh. This can still crack but the have had great success on 2 & 3 story Victorians. In some cases it was better to remove the old horse hair oath and plaster and install drywall aka Sheetrock. Going over brick is the same it is based on what is holding everything together, not much more than horsehair on brick expect cracks. Many Victorians used wall paper to hide the cracks. After remodeling many of these grand old houses we found new Sheetrock and wallpaper gave the best return on investment while maintaining the historic tax exempt status.

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  • Thanks @Ed Beal. Are you talking about little cracks that appear in the surface less than an hour after the structolite starts setting? They look like a dried up lake bed. – dwilli Sep 22 '19 at 2:54
  • Little cracks and large when there is a sub surface crack. – Ed Beal Sep 22 '19 at 3:02

After having done a lot more work on this job, I'm thinking it's a combination of a lot of the tips and answers given here. I've had good success with wetting the brick underneath and with mixing the material very dry, with just enough water so it becomes a paste and spreads. When mixing it should easily hold its peaks.

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