I am in the process of applying polyurethane varnish to a finished workpiece and have been sanding between coats as advised by the manufacturer instructions. I initially wondered why this was necessary until, after applying the first coat, I noticed that the wood was bumpy where it hadn't been previously. Why does this happen, (am i doing something wrong?) and surely sanding between coats ruins the finish of the varnish?
Edit: I used this varnish: http://www.woodoc.com/products/24.