I need to plaster a few walls in the basement. One wall is currently being used for practice. I can achieve fairly smooth surfaces using lime-cement plaster. I install the quick plaster guides, apply a scratch coat first, and after some time, I apply the finishing coat. So far, everything is going well.
However, I still need advice on when and how to properly float the plaster. Also, the difference between floating and smoothing is not entirely clear to me. As a general rule for timing, I currently use this guideline: Start when the plaster can hardly be indented with a finger.
A finish coat will later be applied over the lime-cement plaster.
My questions:
1. With what tool and when should I float the plaster? A plastic float or a felt float? Are there possibly multiple stages, for example floating with a plastic float after about 2 hours and then smoothing with a sponge float or felt float the next day (I read something like this)?
2. What exactly is the difference between floating and smoothing?
3. I have often read that plaster should not be floated if tiles will be applied over it later. Why is that?
Thank you in advance for your help.
However, I still need advice on when and how to properly float the plaster. Also, the difference between floating and smoothing is not entirely clear to me. As a general rule for timing, I currently use this guideline: Start when the plaster can hardly be indented with a finger.
A finish coat will later be applied over the lime-cement plaster.
My questions:
1. With what tool and when should I float the plaster? A plastic float or a felt float? Are there possibly multiple stages, for example floating with a plastic float after about 2 hours and then smoothing with a sponge float or felt float the next day (I read something like this)?
2. What exactly is the difference between floating and smoothing?
3. I have often read that plaster should not be floated if tiles will be applied over it later. Why is that?
Thank you in advance for your help.
A
AallRounder2 Jan 2010 08:31Tips from an Amateur Plasterer
I’m not a professional, but I have manually plastered two entire houses inside and out, and a third only inside (outside: facing brickwork). I have worked with insulating plaster, cement plaster (basement plinth), lime-cement plaster, and gypsum plaster. So, I have some experience.
In my opinion, your working technique is fine to achieve at least 15mm (0.6 inches) total plaster thickness. The plaster mortar should achieve this in a single layer. If you want to apply an additional scratch coat, the surface of the lime-cement plaster shouldn’t be too smooth; in my view, don’t polish it too much.
Regarding point 1:
After the scratch coat, you probably throw on the rest, then level it off with an aluminum straightedge over your fast plaster profiles and fill any remaining holes. When applying a single coat, I now let the wall rest for a few hours (while working on the next one). Once the plaster has set enough so it doesn’t come off when rubbed but can still be spread easily (elastic modulus), I take the largest plastic float possible and smooth out all transitions and small surface holes. During this, you remove excess mortar and redistribute it where it’s missing. Your timing is correct if the plaster sticks, doesn’t sag, and after setting, there are no hollow spots.
In my opinion, you can completely skip the steel troweling because of the scratch coat. A too-smooth surface worsens adhesion.
Regarding point 2:
I can only answer from my understanding. A professional might provide better definitions. For me, “rubbing” means the usual final smoothing with a plastic or wooden float to finish the top (or only) plaster layer. The goal is to smooth transitions and small surface holes. The big risk is “overworking,” i.e., rubbing so much that the plaster starts to crumble. Even in the top layer, enough binder must remain! For lime plasters, I personally rub with lime water to improve this process.
“Smoothing” means creating a final surface finish, in my view. I would equate smoothing with troweling because it only becomes truly smooth with a trowel. For lime, lime-cement, and gypsum plasters, I trowel with added water on the wall and the trowel to get a mirror-smooth surface. For gypsum plaster, I use a sander again after drying.
Regarding point 3:
Do you really mean “rubbed” and not “smoothed”? I work by the basic rule that subsequent layers of any type require a slightly rough surface to avoid unnecessarily worsening adhesion of the next layer. I always rub (as above) the plaster to produce a uniform surface.
Maybe you can use some of my amateur tips, or maybe an expert will say that I’m wrong. Then I will also learn something new.
I’m not a professional, but I have manually plastered two entire houses inside and out, and a third only inside (outside: facing brickwork). I have worked with insulating plaster, cement plaster (basement plinth), lime-cement plaster, and gypsum plaster. So, I have some experience.
In my opinion, your working technique is fine to achieve at least 15mm (0.6 inches) total plaster thickness. The plaster mortar should achieve this in a single layer. If you want to apply an additional scratch coat, the surface of the lime-cement plaster shouldn’t be too smooth; in my view, don’t polish it too much.
Regarding point 1:
After the scratch coat, you probably throw on the rest, then level it off with an aluminum straightedge over your fast plaster profiles and fill any remaining holes. When applying a single coat, I now let the wall rest for a few hours (while working on the next one). Once the plaster has set enough so it doesn’t come off when rubbed but can still be spread easily (elastic modulus), I take the largest plastic float possible and smooth out all transitions and small surface holes. During this, you remove excess mortar and redistribute it where it’s missing. Your timing is correct if the plaster sticks, doesn’t sag, and after setting, there are no hollow spots.
In my opinion, you can completely skip the steel troweling because of the scratch coat. A too-smooth surface worsens adhesion.
Regarding point 2:
I can only answer from my understanding. A professional might provide better definitions. For me, “rubbing” means the usual final smoothing with a plastic or wooden float to finish the top (or only) plaster layer. The goal is to smooth transitions and small surface holes. The big risk is “overworking,” i.e., rubbing so much that the plaster starts to crumble. Even in the top layer, enough binder must remain! For lime plasters, I personally rub with lime water to improve this process.
“Smoothing” means creating a final surface finish, in my view. I would equate smoothing with troweling because it only becomes truly smooth with a trowel. For lime, lime-cement, and gypsum plasters, I trowel with added water on the wall and the trowel to get a mirror-smooth surface. For gypsum plaster, I use a sander again after drying.
Regarding point 3:
Do you really mean “rubbed” and not “smoothed”? I work by the basic rule that subsequent layers of any type require a slightly rough surface to avoid unnecessarily worsening adhesion of the next layer. I always rub (as above) the plaster to produce a uniform surface.
Maybe you can use some of my amateur tips, or maybe an expert will say that I’m wrong. Then I will also learn something new.
Tips from the DIY Plasterer AW
Hello "DIY Plasterer",
Now that’s what I call perfect help. My mistake was probably that I only worked with the float. The only question I still have for future tasks is: If I want to achieve a really smooth surface, for example for wallpapering, when should I start floating?
So, after the plaster has set to a certain degree, should I rub it with the plastic float and then wait a few more hours, or float it immediately afterward?
And again, many thanks for your answer. It certainly gives me a lot of confidence and reassures me that I’m not doing everything wrong. ))
Hello "DIY Plasterer",
Now that’s what I call perfect help. My mistake was probably that I only worked with the float. The only question I still have for future tasks is: If I want to achieve a really smooth surface, for example for wallpapering, when should I start floating?
So, after the plaster has set to a certain degree, should I rub it with the plastic float and then wait a few more hours, or float it immediately afterward?
And again, many thanks for your answer. It certainly gives me a lot of confidence and reassures me that I’m not doing everything wrong. ))
A
AallRounder2 Jan 2010 11:54Hello Mari, I’m glad to hear it helps you,
using only the plastering float was probably frustrating. The plastic floats are for "rubbing" until they are only 1 mm (0.04 inches) thick and break. I’ve already worn out dozens of them.
I had to replaster almost all the interior walls (with insulating plaster for brickwork) and repair many square meters of others. The walls weren’t smooth enough for me, so I applied gypsum plaster again, wall by wall. I will do the window walls with lime finish in spring, as I don’t have moisture problems there. Gypsum absorbs moisture, which is not ideal for exposed walls. Since I have facing brickwork and will only do joint repairs in a few years, capillary moisture can penetrate inside. A 20 mm (0.8 inches) interior plaster layer can handle this, but not if it contains gypsum!
Regarding the floating process: if you do a whole wall measuring 6 x 3 meters (20 x 10 feet), it gets stressful. No drafts, windows closed, keep the air humidity as high as possible. First “scratch” the entire surface with the trowel, then very quickly and continuously go back to the start and spray water from top to bottom so the water runs well. Hold the float with the felt horizontally facing up, spray plenty of water on it, then just before reaching the wall, rotate it 90° and press it onto the wall! Make nice circular motions and smooth out all trowel marks. Do not open tiny holes! In my experience, the right timing when working with gypsum is before the spots turn dark. I noticed with small areas that I could start floating after a very short time using less pressure. It worked best with an application thickness of about 6 mm (0.24 inches) and 65% relative humidity at 10 °C (50 °F) outside temperature. Under these conditions, I was still able to float the beginning well on a 20 m² (215 ft²) wall after 3 hours.
Of course, you can also float the ready-mixed plaster. But will it be as smooth as gypsum? You can sand the gypsum plaster, which really results in a smooth finish! If you use ready-mixed plaster, choose a breathable primer before wallpapering, preferably diluted to avoid sealing the walls.
With lime plaster, you can float better already. After it dries, an additional layer of lime finish is applied, left to set a bit, and then smoothed with the float as described above for gypsum. In my opinion, a separate lime finishing layer is better than floating the second coat of lime mortar, because rubbing can cause the binder to “disappear,” so you have to be cautious.
Alternative clay plaster:
I’m not a “clay person” and watch the conflicting debates between plasterers and clay enthusiasts with dismay. This only scares away newcomers to clay plaster. I stick to “normal” plaster because our house never had clay and I’m allergic to the wood chips often included. However, feel free to explore this alternative; my few tips only refer to cement-, lime-, and gypsum-based plasters.
Best regards
using only the plastering float was probably frustrating. The plastic floats are for "rubbing" until they are only 1 mm (0.04 inches) thick and break. I’ve already worn out dozens of them.
I had to replaster almost all the interior walls (with insulating plaster for brickwork) and repair many square meters of others. The walls weren’t smooth enough for me, so I applied gypsum plaster again, wall by wall. I will do the window walls with lime finish in spring, as I don’t have moisture problems there. Gypsum absorbs moisture, which is not ideal for exposed walls. Since I have facing brickwork and will only do joint repairs in a few years, capillary moisture can penetrate inside. A 20 mm (0.8 inches) interior plaster layer can handle this, but not if it contains gypsum!
Regarding the floating process: if you do a whole wall measuring 6 x 3 meters (20 x 10 feet), it gets stressful. No drafts, windows closed, keep the air humidity as high as possible. First “scratch” the entire surface with the trowel, then very quickly and continuously go back to the start and spray water from top to bottom so the water runs well. Hold the float with the felt horizontally facing up, spray plenty of water on it, then just before reaching the wall, rotate it 90° and press it onto the wall! Make nice circular motions and smooth out all trowel marks. Do not open tiny holes! In my experience, the right timing when working with gypsum is before the spots turn dark. I noticed with small areas that I could start floating after a very short time using less pressure. It worked best with an application thickness of about 6 mm (0.24 inches) and 65% relative humidity at 10 °C (50 °F) outside temperature. Under these conditions, I was still able to float the beginning well on a 20 m² (215 ft²) wall after 3 hours.
Of course, you can also float the ready-mixed plaster. But will it be as smooth as gypsum? You can sand the gypsum plaster, which really results in a smooth finish! If you use ready-mixed plaster, choose a breathable primer before wallpapering, preferably diluted to avoid sealing the walls.
With lime plaster, you can float better already. After it dries, an additional layer of lime finish is applied, left to set a bit, and then smoothed with the float as described above for gypsum. In my opinion, a separate lime finishing layer is better than floating the second coat of lime mortar, because rubbing can cause the binder to “disappear,” so you have to be cautious.
Alternative clay plaster:
I’m not a “clay person” and watch the conflicting debates between plasterers and clay enthusiasts with dismay. This only scares away newcomers to clay plaster. I stick to “normal” plaster because our house never had clay and I’m allergic to the wood chips often included. However, feel free to explore this alternative; my few tips only refer to cement-, lime-, and gypsum-based plasters.
Best regards
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