Plaster is Only as Good as Your Preparation: Avoiding Disaster

Base coat of plaster applied in a shower, only to find out that the taped seams were all compromised due to improper prep.

I just finished the initial coat of plaster in a plaster shower, when I was getting ready to do a second leveling coat the next day. I'm alway excited as I draw closer to being able to start putting up finish coats. The cool thing about this particular job, was that I was going over some new seam tape developed by Custom for the their Red Guard product. The felted tape "seams" (see what I "dad" there? Oops did it again) to be their solution to creating a truly waterproof shower surround using cement board as a base. I was excited because the prepared seams looked so beautiful and smooth. Unlike using a fiberglass mesh they looked amazing. Nothing against fiberglass mesh I use it all the time and it works perfectly. However, this looked so good. I was admiring the soft round over radius going into the soap dish, getting ready to apply a second coat, when some muse tapped the back of my plaster artisan brain and said, "I wonder what they packed behind this round over?" The substate is entirely cement board and Red Guard, no thin set. Maybe it was the gap at the bottom of the wall where plaster should meet tile that made me second guess the quality of the prep.

Prepping for shower plaster is not prepping for tile. The gap left here gave me some extra work to do, while normally the tile would cover it.

Well I pressed on the perfectly beautiful radius I was in the middle of helping create and it just caved. The guys had prepped poorly, even for a tile installation. Rather than making sure the tape was properly backed they were going to rely on rigidity of the tile to make up for quick and sloppy install. I pressed on all the seams and the same result.

Pressing on the tape seams, I found the prep guys had failed to adequately back the tape at the seams.

I was ecstatic that I caught this problem when I did. It could have been the customer calling me after trying to clean their shower the first time with failing plaster, or worse. Rather than take any chances, I checked every inch of every seam, and here was the result.

All the seam tape had been improperly installed, with air gaps behind it and no Red Guard between the tape and the cement board.

I ended up repairing the seams by using fiberglass mesh and an elastonumeric primer. Then I proceeded by applying a base coat to the seam to level it up to the surrounding base coat. At that beautiful radius, well I had to back fill the radius with thinset and fiberglass mesh to provide a hard stable surface for the plaster to adhere to. Down below at the gap, I also had to pack with mesh and thinset to create a hard stable surface that I could plaster over. When Plastering over a board product, whether sheetrock, cement board or what have you the plaster is really just a veneer and is relying on the structural rigidity of the backing board. In this instance I caught it early and was able to correct before I got in too deep.

Finished shower plaster.

One other quick preparation bump in the road. Not every wall is level. In fact, almost no wall is level. Working with a designer to have a plaster wall meet flush with an installed procelain slab on a wall. The porcelain slab was 1/4" thick, and I began the job knowing I'd need to build up my plaster about an 1/8" to meet nice and flush. Reality struck:

In this picture you see the porcelain slab installer did a great job leveling the wall for his porcelain. This meant I need to build up 1/2" of plaster to meet the face of the porcelain flush.

The porcelain installer had already ran into and corrected for the issue for their install. The wall was concave by as much as 3/16" and they had to level the wall to correctly install the porcelain slab. To get my plaster to meet up flush, I ended up having to build up the wall with a larger aggregate plaster before I could proceed with veneer coats.

The wall behind this porcelain slab and plaster had to be built up 3/16" plus to get it level.

The end result was beautiful, however it took quite a bit more material and labor to get there, because the existing wall was so badly concave.

So, when thinking about plastering and how beautiful it is, remember, the finished product is completely reliant upon the preparation of the substrate, easy to say, but as with many worthy endeavors in life, the devil is in the details.