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[Cob] Finish Coat Help Needed

Shannon Dealy dealy at
Wed Mar 12 01:50:59 CDT 2008

On Wed, 12 Mar 2008, Stephen Karrington wrote:

>> Without knowing exactly what went in to this finish, any recommendation
>> that would work would be a lucky guess.  What exactly is the mixture of
>> this finish coat?
> I'm guessing its clay powder and color. I don't think anything else is
> in it. The only other thing that might be in there would be tapioca
> flour. And oil/wax combo was put on top.

You misunderstood me, I was wanting to know what was in the mix that made 
the surface oily.  You don't mention the type of oil or wax that was 
applied, however, you only really have two options, find a way to harden 
the oils on the surface or remove the finish coat down to a level where 
the oil/wax didn't reach, because anything you attempt to put over the 
surface is unlikely to bind to an oily surface.  If it was me, here are 
things I might try:

   - Talk to a chemist friend of mine about anything I might apply to
     harden the wax/oil (this would require knowing what oil(s) and wax(es)
     were used.

   - Apply high heat to the surface in hopes of possibly polymerizing the
     oils/waxes, this of course runs the risk of igniting the surface and
     possibly the rest of the building around it.

   - Use a torch and attempt to burn the oil/wax off the surface.  Of
     course this runs an even greater risk of igniting anything in the
     surrounding area.  Fumes might be rather nasty or toxic depending on
     just what was applied.

>> I think this can be readily dealt with using repeated coats of boiled
>> linseed oil, or one of the alternative drying oils such as tung oil.
> I have 1 gallon containers of linseed oil. I don't think it has any
> chemicals in it for drying. I already put one coat of this oil on the
> surface and it didn't do anything. Its as dry and dusty as it was before!

This may mean you simply didn't apply enough, or the material is so porous 
that it just runs right through.  Generally, you would need to apply a 
number of coats of linseed oil, when making an earthen floor, it usually 
requires four to seven coats to completely fill in and bind the surface,

NOTE: I don't recommend using raw linseed oil for these types of 
applications as it just doesn't dry completely.  There are some non-toxic 
varieties of boiled linseed oil.

Shannon C. Dealy      |               DeaTech Research Inc.
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