Paper maps get wet sometimes, and then pages get stuck together and ripped.
the best way to waterproof a map is to laminate it, however its not to flexible. If it doesn't need to be folded or only folded once this is the way to go.
To make a waterproof map that's practical you need to go with this method.
- waterproof sealer (like Thompson Water Seal)
- Find a clean surface to place the map fully open
- Paint a full layer of the sealant over the entire map
- Paint the other side
- hang it up flat and let it dry for over 24h or more
After waterproofing, you map will be oily but it will hold up as well as a paper map in dry weather. I used this method on a one month backpacking trip in Iceland. It rain 5 out of 7 days, and the maps survived quite well. Some problems are the paper will rip, and the corners of the folds wear like normal paper. A way you can solve this is by using clear packing tape on the outside before waterproofing it. (untested)