Lisa,
I am not sure why you think your approach would be a problem. If you are painting discrete areas of the canvas at different times, each area should be just a single/ double layer. If you, for example, are painting hands but covering the whole surface, well that's another question.
If you really are interested in the chemical/technical implications, you might think about posting your question on the Cowdisly site, which is quite focused on materials.
Here's the url:
http://groups.yahoo.com/group/cowdisley/
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