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Top Coat or Finish coat  RSS feed

 
Posts: 20
Location: NW Arizona
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I have been looking for a finish coat recipe for my RMH the book only tell what not how much,and a lot of it will be going around the bottom 1/3 of barrel. TIA David
 
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Posts: 1257
Location: latitude 47 N.W. montana zone 6A
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David; not quite sure what you're asking for here but... If you want to add some color to your finish you can use concrete dye, it is readily available at any building supply. I mixed red & some black to get a brick color.
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Posts: 219
Location: S.W. Missouri, Zone 6B
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I think what is being asked, is what is the recipe or recipes for the top coat.

Think traditional plaster. You have a rough coat, and you drag that wide toothed trowel over it creating relatively wide ridges. We might think of this as the first layers that are typically used in the cob/adobe techniques. We don't trowel, typically, but poke in finger holes and whatnot to make a nice rough surface to catch the next coat/layer.

Traditional plaster, if I recall properly, then has two more layers, I think the middle layer is called a brown coat, and it adheres to the rough toothed coat, and is finished with a smoother trowel but one that still has a notable tooth to it (creating smaller ridges). Then the final coat is the smooth coat, and goes onto the brown coat.

Note the change in roughness and "tooth" of each layer. Each layer gets smoother, and it sits on top of a rougher surface, so it gets a good tight grip. (Proper curing/drying times will also be important to observe.) There are also recipe changes that I do not recall, but I assume an Internet search would reveal these to those interested.

Applying this to a RMH, as I understand it, some kind of straw like material is added to the top coats (cut, beat, whipped, shredded into small to tiny bits). This makes a binder and adds tensile strength, so that striking the surface with hard objects is less likely to chip. It also helps to reduce stress cracks. I have read somewhere someone (I forget where) that some have used celluse insulation (basically a paper-based insulation, that is run through a chopping machine to shred it, and then it is blown into attics, etc). A "handful a batch" is what I recall reading - whatever that means

Or hair, or using manure in the mix (animals chomp up the straw or straw like fibers).

These fibers also tend to add an insulating value or effect. If temperatures are high enough they may burn out of the mix. The insulation value may remain, because this leaves air gaps. But if too large of gaps it may also weaken the cob/adobe. And obviously, if the fibers burn out they no longer add stress/tensile strength to the mix

Changing amounts of sand to clay ratios can also have an impact (sharp sand is always better in these building situations, because the sharp edges grip the clay better).

I have also read of some people adding traditional plaster ingredients to their final top coat, to bring that to a harder surface finish.


Which leaves the question asked...

What are the recipes for the appropriate layers of cob, and what are the alternates and substitutions?

I don't have that answer off the top of my head. Either search the Internet, this web site, or buy a cob home builders book, or perhaps someone here will provide some answers? I've read these recipes, but I do not recall exactly where, nor do I recall them.
 
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