Failure: The Great Teacher

Ripping off the cracking lime plaster

Scraping off cracked lime plaster from a to-smooth earthen wall

Failure: the omission of expected or required action. I think this is what they call “Another F-ing learning experience”. It’s sometimes necessary to make mistakes to learn what not to do. I thought I’d share mine.

A year ago we plastered a wall with a sand and lime mix. The mix wasn’t very precise and a little thin. It was applied to a wall that had little to no key, nothing for the plaster to hold on to. I have always finished the ‘scratch’ coat with my hands in the past. I guess that left enough of a key or grip, but this was finished with a pool trowel. And huh… not scratched.

Pool trowel ( never leaves my right hand ) Since I got my hands on the pool trowel – I’ve never looked back. It has been permanently implanted in my right hand. Especially while working with the successive layers of mesa dirt. When used with skill, a skill I am still perfecting, it can make a lumpy, uneven wall as smooth as can be. The pool trowel has curved corners so you can work non-flat planes with great success. Curvilinear surfaces like a swimming pool for instance.

Now for the failure. I thought to myself as I was applying the lime plaster – “I wonder if there is enough of a key”… Well, there wasn’t. I had left the wall smooth, with little for the plaster to hold onto. It’s tempting to do. I guess I knew better, but when your looking at a building that has no smooth or finished features, you just want to make something look good! So I ended up smoothing the infill layer, or what was supposed to be the scratch coat. This may have been appropriate if a clay paint or alis were to be used as the finish, but not a finish plaster.

Scarifier From now on I will use the scarifier ( ‘scorifier’ or ‘scratchifier’ ) tool for creating a mechanical key for the finish plaster to hold on to. It’s also been useful for numerous layers of infill plaster, where you can’t fill the area with enough mud, before it gets to heavy and falls off. I apply as much cob / infill as it will take and score it for future applications of mud. After it has set you can wet down the surface and apply more mud. Score it again, if needed and repeat.

It took an evening to remove the plaster from the walls. It came off way too easy. What’s nice is that the wall actually looks pretty good underneath the old lime plaster. Smooth. I washed it all down with a sponge and sponge trowel. Now that’s the way it will stay. I might end up mixing some slip into the wall for a little extra polish, but for now that’s on my someday-maybe list.

Could the failure have been the plaster itself? Yes, though even crappy plaster would have held on longer if it had something to hang on to. Lime plaster is usually applied in 3 coats. A rough sand / lime mix, a lime / sand mix and a lime wash. All the while keeping it wet and allowing it to set before coats. We only applied one thin coat.

I suppose what I’ve learned from this is… they call it the scratch coat for a reason! Take some time to experiment with plasters before committing to future plaster removal.

Any failures you wish to share?

Destorying the plaster Smooth earthen plaster finish

2 Responses

  • andar909 says:

    hi, andar here, i just read your post. i like very much. agree to you, sir.

  • copper says:

    Great quote:
    Architect Kengo Kuma lecturing at the Royal Academy of Arts, being critical of Frank Gehry, mass production and industrialization of architecture: ‘Sushi is a good metaphor for my architecture. The importance in sushi is to choose the best material from the place, in season. ‘If the journey of the ingredients is too long, the taste of the sushi is compromised. That is a problem that can’t be solved by modern technology, and that programme of using local material in season is the secret of good taste, and the secret of my style