In the 80's, the US Forest Products Laboratory in Madison, WI conducted a 
thorough study of vapor barrier properties of a host of coating materials. 
While they did not specifically test B-72, acrylic finishes were included. 
Following are relevant factors for your consideration.  These are from 
memory, as I can not put my hands on the report right now.

1) For virtually all materials, the thicker the coat, the better it was as a 
vapor barrier.  Two coats were better than one, three were better than two, 
etc.

2) Coatings that included pigments generally were better than the same 
material without pigments.  The best pigment was aluminum flake.  In fact, 
traditional types of coatings with this pigment were the best vapor 
barriers.

3)  No material tested was a perfect vapor barrier.  All eventually allowed 
moisture to penetrate through it.  Based upon their protocol which monitored 
the samples for a number of weeks (somewhere around 8 weeks, but I don't 
recall exactly), the best were around 90%-98% effective, the worst actually 
INCREASED the rate of moisture migration (this was linseed oil at 
about -2%).

4) Acrylic resins were in the 2%-6% range for 3 coats, as I recall, so they 
are rather poor vapor barriers.  In the transparent (non-pigmented) 
coatings, wax (melted/dipped) was the best, up in the 90% range.  Of the 
more traditional brush-applied coatings, shellac and polyurethane were the 
best, in the 30%-40% range for 3 coats.  Adding pigments would increase the 
vapor barrier properties.

I would suggest you use melted paraffin (or microcrystalline) wax.  You can 
also use dissolved, brushed wax, but this requires multiple coats to build 
thickness.  If this is not feasible, I would use shellac with flake aluminum 
pigment added.  If this is visually objectionable, you can top coat with 
anything else.  Or if you want a coating only, I would suggest shellac, with 
a minimum of 3 coats.  You can get flakes (dissolve in alcohol) that are 
called super blonde, which is a very light yellow, if the orange color is a 
problem.  Do not use white shellac, as it is bleached to remove the color, 
and some residues remain, as well as the shellac molecule is damaged and I 
don't know if it will perform the same as regular shellac.

Marc

American Conservation Consortium, Ltd.
     4 Rockville Road
     Broad Brook, CT 06016
     www.conservator.com
     860-386-6058

*Collections Preservation Consultation
*Conservation Assessments & Surveys
*Environmental Monitoring & Low-Tech Control
*Moisture Management Solutions
*Collections in Historic Structures
*Collections Care Grant Preparation
*Conservation Treatment of:
     Furniture
     Painted Wood
     Horse-Drawn Vehicles
     Architectural Interiors

Marc A. Williams, President
     MS in Art Conservation, Winterthur Museum Program
     Former Chief Wooden Object Conservator, Smithsonian Institution
     Fellow, American Institute for Conservation (AIC)


----- Original Message ----- 
From: "Dan Bartlett" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 19, 2007 8:56 AM
Subject: -- Spam --[MUSEUM-L] B72 as Vapor Barrier


> Hey all:
>
> Will B-72 act as a vapor barrier?  I need to drive some screws through the 
> laminate inside a sealed exhibit case.  I'd like to seal the recesses with 
> 2-3 coats of B-72.
>
> Thanks,
>
> Dan
>
> -- 
> Dan Bartlett, BFA, MA
> Curator of Exhibits
> Midway Village & Museum Center
> Rockford, IL
>
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