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