I've had problems sometimes with PVA and similar acrylic mediums creating air pockets when mounting canvas. If the wood is sized properly it seems to work better. Hide glue is a traditonal method that works quite well, but for a couple years now I've been using a dextrine starch paste product called Yes! that is extemely simple to use and holds the canvas very well.
http://www.ganebrothers.com/products.../yes_paste.htm
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