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Second-guessing skim-coating

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Second-guessing skim-coating

Postby steste » Mon Jan 28, 2013 11:22 pm

We've already skim-coated our bathroom walls with Easy Sand setting-type joint compound, but poking around the web a little more, it looks like that might not be the best option. Our condo is in a Boston suburb triple-decker built circa 1890. The walls are single-coat horsehair plaster (I assume lime) on wood lath.

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The bathroom wall had paint under the wallpaper, so we primed over it with an oil primer before skim-coating. The rest of the house looks like this photo after taking the wallpaper off, just a single bare 3/8" coat. My understanding is that wallpapering over a single coat was a way to save money.

We would like smooth, painted walls. I guess my questions are

1) Would gypsum plaster (Diamond finish) give a better end result than Easy Sand (assuming I did it right)? I like the idea of not having to sand, the material is cheaper, and I could pick up a skill that a lot of people are afraid of.

2) With gypsum plaster, could I get away with not using a PVA bonder? I've read that PVA acts as a vapor barrier, more so than gypsum plaster.

3) How much more difficult and/or expensive would it be to go whole-hog and put up a top coat of lime plaster? Would it adhere well to the existing wall? I imagine there are suppliers for the appropriate materials in the Boston area.

Thanks for your time.
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Re: Second-guessing skim-coating

Postby Texas_Ranger » Tue Jan 29, 2013 8:21 am

1) In terms of finish quality I don't think there will be much of a difference. However, joint compound contains a whole bunch of additives that some people consider harmful for old walls. I have to admit I use it, at least in dry locations.
2) You don't need the PVA anyway. Just moisten the old plaster to keep it from sucking the water out of the new plaster too quickly. With joint compound, I don't even pre-moisten.
3) Regular lime plaster won't, the surface is too smooth. If you manage to find it somewhere, you can use a mix of lime putty and powdered marble for a perfectly smooth finish.
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Re: Second-guessing skim-coating

Postby Sombreuil_Mongrel » Tue Jan 29, 2013 9:42 am

We learned by trial & error to use gypsum finish coat over gypsum base coat, lime finish coat over lime plaster.
If you opt to put gypsum over lime, Isolate them from each other with bonding agent, even though this alters the way the wall breathes.
You don't have 1-coat plaster. You have a smooth, sanded finish coat. You can buy ready-to mix sanded finish coat, or add fine plastering sand to your white coat and match the finish perfectly.
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Re: Second-guessing skim-coating

Postby steste » Tue Jan 29, 2013 9:28 pm

Good catch, Casey, I never noticed the thin finish coat.

Ranger, what you're describing sounds like a recipe for polished plaster. If I picked up some Oldcastle or USG Ivory type S hydrated lime and made some putty, and mixed it with marble athletic field chalk, you're saying I could skim-coat with that over the lime finish coat? Any idea how durable and/or paintable this surface would be? Sounds like it could end up very white by itself, and I could either prime and paint the unpolished surface or polish and wax the surface. I suppose a flat finish would hide imperfections better, so painting would be the way to go unless I found a way to flat-finish polished plaster.
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Re: Second-guessing skim-coating

Postby plastrr385 » Tue Jan 29, 2013 9:36 pm

I would bond the walls first and use lime and gauging ( you can add sand if you wish ).
If you wish to add sand use silica sand and mix it 100lbs lime to 100lbs sand. you should still use gauging to make it hard and you can add some retarder to the lime water to slow the set of the gauging down. Mix approx 5/8 bucket of lime place about 1-2" of water on the lime and put about 3-4 scoops of gauging to the water and mix till all the gauging is incorperated into the lime. They also make quick and slow set gauging. I make my scoops out of windshield washer fluid bottles. Cut it about 2" below where the handle stops. Hope this helps
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Re: Second-guessing skim-coating

Postby ripfish » Tue Jan 29, 2013 10:01 pm

Steste, If your house was built in 1890, your plaster walls are probably horse hair lime plaster. You should consider using natural lime plaster products, such as those sold by Virginia Limeworks, Lynchburg Virginia (Google on the web and they have a good website). I can see in your picture some horse hair. If your repair is to repair a complete hole in the wall, you will need to first replace the lath, then start applying the lime plaster. I did extensive lime plaster repairs on my 1731 home, and learned how to do it by attending free in-house clinics at Virginia Limeworks. I know this is quite a ways from Boston, but they can teach you everything you need to know in an 8-hour session and would be a great trip. You will learn how to (1) mix and apply horse hair plaster to the lath, (2) mix and apply brown coat, and (3) mix and apply the finish coat. If you have an exposed hole as you have, you will have to do all three. Virginia Limeworks sells natural lime plaster products. I have many pictures of the process, step by step from my work in my home. From my experience, if you have old plaster it should have first a brown coat applied, then a finisht coat. I don't think, however, that you can learn how to do it correctly without hands-on training. I learned everything I needed to know about lime plaster from Virginia Limeworks, but at first I was slow and messy. In time, I became very good after a few weeks of practice. I have attached some pictures of plaster repair from my home of lath replacement, scratch coat, brown coat, and finish coat. Good luck. Ripfish.
Bedrm before 1.jpg
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Bedrm before 1.jpg
Bedrm before 1.jpg (90.09 KiB) Viewed 10417 times
Bedrm before 1.jpg
Bedrm before 1.jpg (90.09 KiB) Viewed 10417 times
I see that some of my pics are repeated, and out of order, but I hope you see the basic results.
Attachments
MBdrm After plaster 3.jpg
MBdrm After plaster 3.jpg (33.66 KiB) Viewed 10416 times
Plaster Bdrm.jpg
Plaster Bdrm.jpg (84.54 KiB) Viewed 10418 times
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Re: Second-guessing skim-coating

Postby Texas_Ranger » Wed Jan 30, 2013 8:39 am

I'd definitely recommend a flat finish and advie against oil, acrylic or latex paint on plaster! Period appropriate paints are milk paint, distemper (also known by the brand name Kalsomine, sometimes spelled calcimine) which is basically cellulose glue with powdered chalk, mineral paint (rarely used indoors) or a simple whitewash (lime putty thinned with water). I am thoroughly fond of distemper as it's extremely simple to remove if the need arises. Of course this is both a blessing and a curse because you have to remove it before you can repaint or wallpaper (you can usually paint it over once or twice, but as the glue ages, it seems to lose adhesion and if the old paint gets wet again it starts to blister and fall off). On average, you can expect a complete scrape and wash every 20 - 30 years, depending on paint quality. I recently painted over two coats of distemper (one from 1991 and the other from 2004) without any trouble.
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Re: Second-guessing skim-coating

Postby ripfish » Wed Jan 30, 2013 1:27 pm

I just visited "www.virginialimeworks.com". They now have several Utube videos on plaster "how-to" which are very helpful. They also have recommended mixing instructions for various lime products, including hydrated lime formulas and natural lime plasters, and goat hair for the lime plaster scratch coat. They sell these products: some are pre-mixed, called "Mix and Go". This company did the restoration of Thomas Jefferson's Monticello, and are the best and most knowledgeable on the East Coast. Recommend you visit their site and browse around, looking at videos of interest to your situation to help you. They can definitely help with skim coat ideas. They also sell lime paints in historical colors.
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Re: Second-guessing skim-coating

Postby OldTownHome » Mon Feb 11, 2013 9:49 pm

We didn't know the difference when we first started working on our house and ultimately covered everything with skim coats of setting type joint compound (brown bag). I got pretty good at getting it on nice and smooth, and the end result is pretty fantastic, but I now know better. I know our walls and ceilings are a mix match hodge podge of horse hair plaster and sheetrock where plaster was patched. I wish I had known better. But the stuff we skimmed with has held up really nicely for 10 years now, and hopefully for much longer. The next house we work on I'll surely be doing the right stuff. Now I know, and knowing's half the battle. G.I. JOE!!!
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Re: Second-guessing skim-coating

Postby steste » Wed May 08, 2013 10:43 am

I picked up some Virginia Lime Works Building Lime 150 and some fine marble dust in an attempt to skim coat the existing lime plaster wall with matching materials without reducing the water permeability by much. As predicted, I'm having trouble getting the new lime plaster to both stick and be smooth.

On my first try, I put a thin coat onto a piece of luan plywood I had sitting around. It worked pretty well. First, I sprayed the board with water. I mixed 1:1 lime to marble dust by powder volume into water until it was the consistency of mayonnaise. I put it on very thin. After 3 hours, I sprayed it down and troweled it almost smooth, though it did start to come off, so I stopped troweling. I figured that with a second coat, I would be able to get it perfectly smooth and polished.

Emboldened by this success, I soaked my wall the day before and again right before applying the skim coat. I think my mix was a little too thick. I added a bit of liquid soap because someone told me it made the troweling easier. The plaster became crumbly before I finished troweling it, so I just put the skim coat up quickly and left the trowel marks alone, hoping I could come back and fix it after it started to dry and set. Spraying and troweling the plaster before it's ready for touch-up just makes it wet and crumbly, while spraying and troweling after a day doesn't do much, especially for the bigger imperfections from mixing the plaster too thick. It seems very sensitive to timing and moisture level, and with me not wanting to work on it at 3am, I must have missed that window on the first try.

The plaster dried very hard with no cracking in about two days, but it was uneven, so I scraped it off. It basically fell off the wall where it was more than 1mm thick, while the thinner spots had to be scraped and chiseled off. Both the Virginia Lime Works plastering instructions and the Master of Plaster product data sheets say to apply in multiple "lifts" or coats that are as thin as possible.

For my second try, I wet down the wall again and applied a very thin coat of plaster that was closer to the first, wetter consistency. It dried out and crumbled even faster than my first attempt, despite mixing the plaster with more water. It had been a few days since the first attempt, and I think the wall needed more saturation. Also, trying to plaster that thin of a layer was difficult, leaving ridges perpendicular to the troweling direction. I went ahead and applied a second coat about an hour after the first, and the result was a much flatter, smoother coat, though it was still very finicky. I will test the adhesion when it hardens in a few days.

I also skimmed an area on the opposite hallway wall, which the previous owner repaired with drywall. The Master of Plaster product says it will stick to paint, and as I understand it, MoP is basically what I'm making, just with hydraulic lime instead of aged lime putty, and with no additives, while the MoP product has mica powder plus some other mineral called palygorskite and some other secret stuff. The skim coat over paint just turned to dust where it was very thin, which could be due to over-troweling, I don't know. I'll see about the thicker spots soon.

I'm encouraged by a few things. The areas with very thin plaster did stick. If I could get the first coat up fast, thin, and decently smooth, then the second and third coats would even out the remaining imperfections. Each coat does handle a little bit of troweling before it's completely set and a little more troweling later on, as long as you don't do too much at once. The wall could probably stand some more soaking with water, with a final spray-down right before plaster application.

Worse come to worst, I could just PVA the wall and then lime plaster, since it would still have better permeability than the painted wallpaper we just removed. It would be nice if there were a bonding agent that was water impermeable when applied and water permeable thereafter.

I'll keep trying things out. Any helpful hints on wall prep, plaster mixing, troweling, smoothing, polishing, and timing in general? Does a wood float work well on hydraulic lime? Is lime putty any better? Would it help to soak the wall with lime water rather than regular? Any additives I can use to improve the spread-ability, those of you with Venetian plaster experience? If I can manage to figure out a consistent method for an economical skim coat of new lime straight onto old lime, I imagine that would be helpful to someone.
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