The cloth doesn't get you anything, and adds to the chance of getting a fiber of some sort on the O-ring. Your best bet is something like this:

Start by removing the o-ring from the its flange. Then clean the flange with a cloth. Towels are ok, but something less fibrous would be better. If you want to spring for the $$, get a "Pentax Cloth" at a photo shop. Ask for it by that exact name, and they should know what you want. It's a gray polyester cloth about 6"x8" in size, and is designed not to emit loose fibers. These things are designed for wiping off lenses with zero chance of scratching the lens. Anyway, once the door has been cleaned, set it down somewhere, face up, where it won't get any dirt on it.
Dip the o-ring in some fresh water (the camera rinse bucket is OK, but the mask rinse bucket is not) and whip the o-ring in air to remove the water. This process will clean off any sand, hairs, small particles, etc.
Put a small dab of o-ring grease on your thumb. By "small" I mean a squirt about the size of a rice grain. Pinch the o-ring lightly between thumb & forefinger, and pull the entire o-ring through the pinch. Better yet, pull it through a couple of times, twisting gently as you pull. This will smear the grease along the entire length of the O-ring.
proceed directly to mount the o-ring on your housing door. DO NOT put the o-ring down for any reason; mount it directly on the door.

That's about it. All told, the o-ring dance takes about 2 minutes to perform.

Good luck,
D

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