Hello, I have a question. I want to apply felt plaster on a green drywall board. I have already noticed when tiling and previously when applying a liquid membrane in the lower area that the gray liquid membrane turns brown because of the green board. If I apply white felt plaster, how will the color show through? I would like the final color to be beige/yellow. Are there primers in yellow that are applied under the plaster?
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AallRounder11 Sep 2013 13:15Hello Apprentice,
What kind of felt plaster are you planning to apply? Basically, you can apply felt finishing to many types of plaster: gypsum plaster, lime plaster, or lime-cement plaster. Is the wall located in an area exposed to splash water?
In my bathrooms, I have double-layered the ceilings with green drywall boards, applied mesh reinforcement over them, and then applied about 10mm (0.4 inches) of gypsum plaster, which I finished to a mirror-smooth texture. Doing this overhead is a delicate job. After it has dried thoroughly, I lightly sand it with abrasive mesh, and that’s it. Nothing shows through, and there aren’t any moisture issues or hairline cracks directly above the shower. With this type of coating, you can skip special additives because the classic method of a plaster base plus a “thick” layer of plaster works well even on drywall.
If you only want to apply a thin layer of some premium plaster, you should strictly follow the manufacturer’s instructions for drywall substrates. In that case, you might need expensive vapor barriers or primers to prevent show-through. However, your details don’t specify this clearly.
What kind of felt plaster are you planning to apply? Basically, you can apply felt finishing to many types of plaster: gypsum plaster, lime plaster, or lime-cement plaster. Is the wall located in an area exposed to splash water?
In my bathrooms, I have double-layered the ceilings with green drywall boards, applied mesh reinforcement over them, and then applied about 10mm (0.4 inches) of gypsum plaster, which I finished to a mirror-smooth texture. Doing this overhead is a delicate job. After it has dried thoroughly, I lightly sand it with abrasive mesh, and that’s it. Nothing shows through, and there aren’t any moisture issues or hairline cracks directly above the shower. With this type of coating, you can skip special additives because the classic method of a plaster base plus a “thick” layer of plaster works well even on drywall.
If you only want to apply a thin layer of some premium plaster, you should strictly follow the manufacturer’s instructions for drywall substrates. In that case, you might need expensive vapor barriers or primers to prevent show-through. However, your details don’t specify this clearly.
I want to apply the lime-cement plaster from Schäfer-Krusemark. Using the spray method, partially by hand, since the roof slope runs past the shower. Normally a plasterer would do this work, but he hasn’t been on site yet, and since I’ll be providing the materials, I want to gather some information in advance about what I’ll need. The application thickness will be around 3–5 mm (0.1–0.2 inches), possibly thicker on the dormer cheeks, but not near the window. So, you mean the manufacturer should have information on whether a primer is necessary? I’ll check that out, thanks for the info.
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AallRounder11 Sep 2013 19:47In my opinion, this is likely to cause problems. The manufacturer requires a minimum application thickness of 15 mm (0.6 inches), with only a few areas allowing 10 mm (0.4 inches). Using 3-5 mm (0.1-0.2 inches) won’t work. A special substrate treatment is only described for wood wool panels (HWL), but for gypsum boards (GB) there should be something similar. It’s best to check with the manufacturer. The technical datasheet is easy to find online.
But what is the point of applying render on gypsum board in areas partially exposed to splashing water? Although I don’t have gypsum board walls but solid walls, I use lime-cement base coat plaster and white lime finish plaster. In splash water areas, I have sealed heavily with two layers, carefully incorporating sealing tapes, and then tiled everything. Millimeter-thin plaster in the splash water zone — especially on gypsum board — won’t bring you any joy. Who designed this?
But what is the point of applying render on gypsum board in areas partially exposed to splashing water? Although I don’t have gypsum board walls but solid walls, I use lime-cement base coat plaster and white lime finish plaster. In splash water areas, I have sealed heavily with two layers, carefully incorporating sealing tapes, and then tiled everything. Millimeter-thin plaster in the splash water zone — especially on gypsum board — won’t bring you any joy. Who designed this?
Somehow, Schäfer-Krusemark lists "Filzputz 600" as a thin-layer lime-cement plaster, and I can’t imagine it needs to be applied at 10-15mm (0.4-0.6 inches) thickness. There is also a primer from Krusemark designed to prevent "bleeding through on gypsum fiber boards," according to the manufacturer's information. It’s not a direct spray area; I was rather thinking about rising water vapor. A shower enclosure will be installed.
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AallRounder12 Sep 2013 07:16Hello apprentice,
“Filzputz 600” is definitely different from “Schäfer-Krusemark lime-cement plaster.” Using the exact product name is always helpful when asking technical questions in the forum.
But again, I see problems: according to the manufacturer’s instructions, the felt plaster should be applied on a mineral substrate. A gypsum plasterboard is anything but mineral. They recommend either their base coat or their insulation system as the substrate. However, even there, the felt plaster is not applied directly onto the insulation boards that the poor houses are covered with. The required layering sequence is: reinforcing mortar – reinforcement mesh – primer – finishing plaster. Only then can you apply it on styrofoam or mineral wool. So it’s anything but a simple, millimeter-thin application. Calculate what all of that costs!
Before the “plasterer” without gypsum shows up at your place, you should download the technical datasheets and make sure he provides a quote that follows these specifications. If he wants to apply the plaster directly on a primer, you should intervene.
Unfortunately, I don’t know how your bathroom is constructed, how big it is, what ventilation options exist, etc. But if it’s an “average” bathroom of maybe 6–8 sq m (65–86 sq ft), in the attic where everything above is sealed with vapor barriers, and in the worst case no window, then prepare for a lot of vapor buildup. In my walk-in, floor-level shower (no enclosure or splash guard), I easily vapor-saturate about 12 sq m x 3.60 m (129 sq ft x 12 ft) height when showering extensively, if the window is closed. The vapor rises, is usually slowed down by a vapor retarder, and buffered in the ceiling plaster (that also includes 12.5 mm (1/2 inch) plasterboard). The excess vapor hits the glass and tiles, which serve as condensation surfaces. The rest seeks out other surfaces, like plaster. If the plaster is very thin, it can't buffer moisture. It becomes saturated just from the vapor, causing material stresses in the plaster. That’s why the plaster needs to be formulated and reinforced. How well the system actually works becomes apparent after a year of bathroom use. Light-colored plasters can discolor with daily vapor exposure (e.g., yellowing, dark spots that don’t go away) or even flake off. The vapor load is especially challenging in winter because you would actually need to ventilate thoroughly to remove the moisture. If this doesn’t happen, vapor lingers in all corners for a long time and inevitably finds microscopic holes and cracks in your vapor barriers, where it causes damage in mineral wool and similar materials that are hard to detect.
In small bathrooms, I would avoid plastering near the shower altogether, seal everything with a flexible system, and tile consistently, especially if it involves a gypsum plasterboard system. Then you have a designated condensation surface where the condensate eventually drains onto the tiled floor and into the floor drain.
“Filzputz 600” is definitely different from “Schäfer-Krusemark lime-cement plaster.” Using the exact product name is always helpful when asking technical questions in the forum.
But again, I see problems: according to the manufacturer’s instructions, the felt plaster should be applied on a mineral substrate. A gypsum plasterboard is anything but mineral. They recommend either their base coat or their insulation system as the substrate. However, even there, the felt plaster is not applied directly onto the insulation boards that the poor houses are covered with. The required layering sequence is: reinforcing mortar – reinforcement mesh – primer – finishing plaster. Only then can you apply it on styrofoam or mineral wool. So it’s anything but a simple, millimeter-thin application. Calculate what all of that costs!
Before the “plasterer” without gypsum shows up at your place, you should download the technical datasheets and make sure he provides a quote that follows these specifications. If he wants to apply the plaster directly on a primer, you should intervene.
Unfortunately, I don’t know how your bathroom is constructed, how big it is, what ventilation options exist, etc. But if it’s an “average” bathroom of maybe 6–8 sq m (65–86 sq ft), in the attic where everything above is sealed with vapor barriers, and in the worst case no window, then prepare for a lot of vapor buildup. In my walk-in, floor-level shower (no enclosure or splash guard), I easily vapor-saturate about 12 sq m x 3.60 m (129 sq ft x 12 ft) height when showering extensively, if the window is closed. The vapor rises, is usually slowed down by a vapor retarder, and buffered in the ceiling plaster (that also includes 12.5 mm (1/2 inch) plasterboard). The excess vapor hits the glass and tiles, which serve as condensation surfaces. The rest seeks out other surfaces, like plaster. If the plaster is very thin, it can't buffer moisture. It becomes saturated just from the vapor, causing material stresses in the plaster. That’s why the plaster needs to be formulated and reinforced. How well the system actually works becomes apparent after a year of bathroom use. Light-colored plasters can discolor with daily vapor exposure (e.g., yellowing, dark spots that don’t go away) or even flake off. The vapor load is especially challenging in winter because you would actually need to ventilate thoroughly to remove the moisture. If this doesn’t happen, vapor lingers in all corners for a long time and inevitably finds microscopic holes and cracks in your vapor barriers, where it causes damage in mineral wool and similar materials that are hard to detect.
In small bathrooms, I would avoid plastering near the shower altogether, seal everything with a flexible system, and tile consistently, especially if it involves a gypsum plasterboard system. Then you have a designated condensation surface where the condensate eventually drains onto the tiled floor and into the floor drain.
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