The quick question - Should I / Can I add a return duct to the basement in order to reduce humidity in the basement?

The details -

I have a "finished" basement. It doesn't feel like one since it was done in the early '60s with panelling and other out dated materials. Some areas of the basement are not finished (eg. exposed foundation), like the laundry room and a closet.

The basement is very humid - at least it is now in the summer. I haven't checked it in the winter. My dehumidifier is filling to capacity and shutting off at least once a day. I empty it before work and when I get home, it is full. When the dehumidifier was not running for a few days, the humidity was 80%. When it is running, the lowest I can get it is about 65%.

One thing I'm surprised about is that I can't find any condensation anywhere. I checked the concrete walls, panelling and the cold water pipes. Nothing.

I am about to remodel my basement and am afraid that excessive humidity will ruin it, although I don't seem to have any problems now (other than the humidity itself). There is no mold or odors.

The basement living area has the laundry room adjoining. The laundry room also contains the furnace and gas hot water heater. The door to this room is always open, so perhaps this is where much of the humidy is coming from? There are two washers and two dryers. Yes, the dryers exhaust to the outside. When I remodel, I plan on putting in a door and keeping it shut.

There is no A/C in the basement - don't need it. There is heat, but not from the furnace. It is actually connected to a heating system from another part of the house. The basement has hot water baseboard.

My furnace has a return run that runs across the basement and through the foundation to the outside. Some people told me this is good because it provides fresh air to the house; others say this is bad because the furnace has to heat freezing air.

Finally, my questions -

1) Keep the return duct to the outside or close it off?

2) Add a return to the basement living area?

3) Why is the humidy so high in my basement?

4) Will adding a return in the basement help with the humidy?

5) Will painting Drylock on the walls help with the humidy?

6) Are there other alternatives to help with the humidy? Should I get another dehumidifier?