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Date: Sun, 03 Oct 2004 10:38:31 -0400
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From: Jim Cameron <toucan@78055.com>
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Subject: MC vs. acetone
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    This discussion of acetone vs. methylene chloride (MC) has come up 
on the LML at various times over the years, and does not seem to be 
something with a definitive answer.  Due to their chemical natures, 
however, MC is inherently superior to acetone as a degreaser, and has 
been used in various industries for that purpose for a long time.  The 
EPA designation as a potential carcinogen warrants some care, but if you 
look closely, many of the things we come into contact with are in that 
same category.  (Check the warnings about carcinogens in gasoline -- 
doesn't stop us from putting it in our cars a couple of times a week, 
right?)  The main concern of their studies has been workers exposed on 
an occupational basis, i.e., more or less daily over a span of years.  
There is also some scientific question about the carryover of the 
typical animal assay to human exposure, but those arguments are too 
complex and ambiguous to bring up here.

    For me the point is that for occasional, low-level exposure, such as 
might be typical of an amateur builder, the risk is more or less nil.  
Choice of gloves is important, as MC will go right through most gloves, 
particularly the latex type we usually wear for doing layups.  If you're 
really concerned, wear a respirator with activated charcoal canisters 
(obtainable from lots of places, like Grainger or McMaster-Carr).  Keep 
the stuff well stoppered, and store it where the fumes are not vented 
into your shop area, in an outside shed, for example.  Five-gallon drums 
are available at any industrial chemical supplier, and one five-gallon 
drum is probably plenty for one airplane project.  Wiping technique is 
important, too:  use a clean rag, change surfaces often, don't hold the 
rag against the container mouth, etc.  Throw-away rags are probably a 
lot better than paper towels, which shed crud all over your layup, and 
may have binders and other contaminants.

    De-greasing seems to be a critical step in bond preparation, so why 
not use the best available means?  (I can understand the Lancair factory 
shop switching from MC to acetone, as they do this every day, often on 
several planes at a time, and may not always have the most careful 
guests assisting them!  They also have a liability situation to think 
about, I suppose.)

Jim Cameron
Legacy N121J