I camp in tents and found a perfect tent sealer.  Thompson's Water
Sealer.  Set tent up outdoors and sponge on, or dip entire tent into 
bucket filled with sealer then wring out and set tent up to dry.  
This really works well.  Spent the night in a 5" downpour rain and 
only about sponge full near the door.  If I had put zipper at top of 
opening I could have proably stayed completely dry.  Zippers should be
treated with a little vasaline to prevent sticking.  This method beats 
the heck out of anything sporting goods dealers have suggested.

Best regards -- Michael Snow