This is a write-up by Daniel Stern a respected lighting guru. Drying them in the oven is the key here. They don't spot. I did mine a few years ago.
"Work with one lamp at a time. Remove the bulbs. Fill the lamp about 1/2 full of warm-to-hot distilled or filtered(!) water, then pour in a splash of "Multi-Surface" Windex or Simple Green cleaner.
Cover the bulb hole with your (clean) thumb or palm, or simply reinstall the bulb to cover the hole. Vigourously shake/slosh/swirl the lamp to agitate the hot soapy water. Do
this over a sink, and hold on with both hands so the lamp doesn't slip
from your grip, fall and break. Then uncover the bulb hole, turn the lamp
bulb-hole-down, and swirl the lamp to cause the water to drain from it in
a circular fashion. Repeat this cleaning step, then rinse the lamp
repeatedly with warm-to-hot distilled or filtered(!) water until all
traces of soapsuds are gone. Hold the lamp firmly with lens facing you and
shake/snap vertically to force out more loose water.
To dry the lamps, place them lens-down in your clean kitchen oven, on the
rack about 1/2 to 3/4 of the oven's height up from the lower element.
Close the oven door. Turn the oven to Bake/350 for 3 minutes, then turn
the oven off and leave the door closed. In about an hour, your lamps
should be thoroughly dry with minimal or no watter spotting -- any minor water spots remaining will be inconsequential to beam performance"
