Sunday, April 22, 2007

Plaster Pictures, As Promised

But first, I want to clear up something about yesterday’s post. I speculated that I had bought two different types of base coat plaster from US Gypsum. After I posted, I went to the USG web site and looked up the product.

The perlited gypsum base coat plaster that I bought last week matched what was on their site. I could not find on the site the bag of perlited gypsum base coat plaster that I had purchased 2 years ago. However, when I downloaded the PDF file that had all of their products and looked up perlited gypsum base coat plaster in that, it showed the bag I had bought 2 years ago, but not the new one. It seems someone in marketing got bored and they just redesigned the bag. The ingredients and instructions for use are the virtually the same on both bags.

Why I have problems with the old product is anybody’s guess. It could just be because of problems inherent with any manufacturing process of a mixed product like this. Maybe the bags I bought two years ago were from the bottom of a mix and they had a little more perlite in them (or not enough). Who knows. Regardless, it seems to be the same product, just different packaging. The site did say there were two versions of the product. One is for lath and the other is for masonry (applying directly over brick or cinder block), but both bags I have indicate it is to be applied over lath.

The old type just sets up so quickly no matter how thin I made the mix. At one point I had it set up on the trowel as I was applying it! I had a quarter inch skim coat of plaster that was hard on the trowel from a mix that was only a few minutes old. This never happened with the new stuff. The working time of the old stuff is 5 to 10 minutes max, while it is 15 to 20 minutes with the new stuff. It is very frustrating to work with. The amount of loss I had in the bathroom was maybe a quart. With the kitchen it was close to an entire bag over the course of the job.

Also, it seems the perlite is a replacement for sand in the scratch and brown coats. They do sell an un-perlited plaster that is to have sand added to it on the job site. I think the perlite puts the “Lite” in Structo-Lite plaster. Being that perlite, once expanded, is an airy, puffed mineral, it must weigh much less than sand. That is not to say that Structo-Lite when mixed with water does not weigh a lot. In fact, it is dense and heavy. I’m sure it’s just lighter than plaster with sand. Oh, those wacky marketers.

Now, on with the show. I should once again point out that I am a novice at plastering. If someone else who seems to know what they are talking about gives you different information, then you should seriously consider taking that advice.

The brown coat went on very well. I mix the plaster in a roughly 3:1 ratio of plaster to water. The instructions give mixing ratios by weight, but I’m not going to weigh dry mix and water. The Red Sea analogy is good, but if given a choice, the scratch coat should be on the dry side and the brown coat should be on the wet site. One batch for me is 6 quarts of dry mix to 2 quarts of water. I mix first with a small hand trowel and then finish mixing with an attachment on a drill. I put up one mix in three batches on the hawk working with a 12-inch metal trowel.

I don’t expect to get it perfectly smooth at first. When initially applying the brown coat to the wall I’m concentrating on thickness. I want to get an even thickness on the wall and get it relatively smooth. I make sure I fill any large voids and get an even thickness at the screeds. Around doors, windows, or anyplace that has trim you should have wooden screeds nailed up to ensure the thickness is accurate. Three eights inch screeds is the standard. I have half inch screeds along the tile, and then 3/8ths every where else. You don’t want wavy plaster where a piece of wooden trim meets the plaster. In the center of a wall though, it’s not as important that it be accurate. This is one of the great things about plaster and old houses. You don’t need smooth, flat walls or perfect angles to start with.

After I have mixed 2 or 3 batches and applied them to the wall I go back over the first batch I put up. This is about 15 or 20 minutes after I applied the first batch. By this time it has the consistency of firm modeling clay. I take a semi-flexible, 4-inch scraper and a squirt bottle and smooth out any areas that came out less than perfect the first time. I fill in any small voids and just kind of clean it up. The next coat is going to be a very thin finish coat, so this is the time to make the wall look good and get rid of any bumps, voids or problem areas. USG says on the bag to leave it rough to except the finish coat. If I was good enough I could probably do the second pass with the 12-inch metal trowel. Maybe someday.

After that I mix another batch and apply it, and then go back and smooth out a batch I put up 2 or 3 batches ago. It goes pretty fast, but does tend to slow down around tight areas. It’s easy to get frustrated around light switches, outlets and other areas like that. Just remember that you have a good 15 or 20 minutes to work with it. I went back to some areas 2 or 3 times and worked on them some more with the scraper and squirt bottle. Once it sets up completely though, you will be reduced to sandpaper, or maybe even a hammer and chisel. This is stuff gets very, very hard when it sets.

Here are some random shots of the room

This is the scratch coat. The walls are very rough.

It’s hard to see but the next two are shots are the same spot on the wall.

This is the first pass.

This is after the second pass with the squirt bottle and scraper.

You can see the wood screeds around the vent.

More shots of the brown coat. This is how it looks now.

Before I started I laid down pieces of cardboard and taped them together. The plaster is much too sticky for plastic or paper. In fact, it’s much stickier than joint compound, but then, why would I be using joint compound on plaster walls (Wink: Gary)

After I’m finished I pull up the cardboard and then once over with the shop-vac. I’ll put down more cardboard for the finish coat.

Next up: The Finish Coat


purejuice said...


Anonymous said...

I commonly use pure gypsum for patch work and fastening stuff in masonry walls (pure gypsum sets in 5-10 minutes, some kinds even in like 2 minutes, to install an electrical box in a masonry wall you just smear some gypsum in the hole, push in the box and hold it until the gypsum hardens). I mostly use the same brand since the hsop where I buy only stocks that one, and the curing time varied greatly. Some bags got rock hard within 2 minutes, others were useable for almost 10 minutes.

Perlite plaster (though cement and lime based) is advertised as "thermo plaster" here since the perlite somewhat acts as thermal insulation. I once used it and the main differences seem to be: the bags are much easier to carry around and once the stuff is cured it's lighter too. On the other hand, why would you need that? Once it's on the wall the difference doesn't really matter any more... besides, thermo plaster doesn't really get hard. The surface can be scratched with the finger nails.

I don't really see any reason to use that stuff, depending on the application I either use pre-mixed bagged lime-cement plaster or mix my own lime plaster.

You can both mix plaster from dry bagged lime or slaked lime and I noticed a big difference. The dry variety is just like sand mixed with water and doesn't get extremely hard. The slaked lime plaster is really sticky and nice to use, it can be troweled perfectly smooth and gets ROCK hard.

Gypsum plaster is pretty rare stuff here, lime plaster is simply cheaper and easier to use (stays soft in the bucket for several hours). I think in old houses gypsum plaster (if at all) was used only for ceilings because gypsum needs to stay perfectly dry (and especially single story brick houses were never completely dry at the bottom). It seems that ceiling plaster didn't contain any sand since it's smooth as glass.

Greg said...

"The surface can be scratched with the finger nails."

This is another one of those things I've read, but which doesn't really apply in real life. If you can scratch the stuff I put up yesterday with you fingernail, then you have finger nails made of steel. The Plaster ceiling medallions I've purchased can be scratched with a nail. From what I've been told, those are made out of Plaster of Paris. They are bright white and have an almost chalky feel to them. I am not putting pure Plaster of Paris on my walls.

"Gypsum plaster is pretty rare stuff here"

Where is "Here"? Are you over in the UK, if so, then what you are saying makes sense. When I first started looking in to plaster I got a lot of information at Period Properties UK. The problem is, the information does not relate well here in the US. Products are different. Things that are easy to get in the UK aren't available here.

This is the umpt-teeth time I've been told to "just go buy slaked lime", and I've yet to be able to find it. Everybody uses different terms: Masons Lime, Slaked Lime, Hydrate Lime, Lime Putty, etc, etc, etc. Trying figure out which is the right – and best – stuff to use for historically accurate lime plaster is very hard, and a bit frustrating.

Ragnar said...

I guess I should start logging into blogger and posting under my real name... you probably know me as Texas Ranger from the oldhouseweb boards.
I am indeed located in Europe and of course, products vary greatly.

As I said, my perlite plaster was NOT gypsum based but cement/lime based and since it was pre-mixed I can't tell the mix ratio - might be pretty bad, lots of perlite to keep the price down. That stuff I used for the basement walls IS fairly soft.

If you want I'll try to look up recipes for slaking lime. I think you have to leave it for months and it's a pretty ugly job. My dad recalls when a cousin of his built a new home in 1964 the threw lime into the unfinished open cesspool and added water. It got hot and started bubbling and splashing wildly.

However, maybe I'm just confusing things once again... might just be mixing the lime with water and let it rest for some time.

German Wikipedia helped a lot... slaked lime is the powder you buy at the hardware store. Slaking is the process I described above, the stuff with the cesspool. Not something you'd want to do yourself... Hydrate lime is the same stuff, and I guess mason's lime too.

For good plaster lime putty is created by letting slaked lime soak in water for... now grab a seat and hold onto your desk or chair... 20 years! The older the better. I vaguely remember typical times of several months.
So, if you manage to buy lime putty that would be perfect, otherwise you're in for some soak time...

Mick said...

ok, I know this is a blast from the past post - but in case you hadn't picked up this info already I thought I'd pass it on....

Gypsum plaster goes "off" as it gets older. When its off it behaves like you describe - it sets way too quick. The reason this happens (as I understand it) is that moisture from the air gets into the unused plaster and the reaction that hardens the plaster partially starts.

Professionals in the UK get around this by keeping their supply in air tight plastic barrels. They will Also throw out old plaster and replace with new. I used to keep an eye on the manufacture date printed on the bag (do they even do that here?) and pick the newer lots, not those from the top / front of the pallets.