Had a similar problem myself.....a quick call t Jim Amos at missleworks resulted in the following advice.
Altimeters can be cleaned with distilled water or any clean water that won't leave hard water residue. Rinse off the altimeter and shake off the excess. Place in the sun or on a dash of car with the windows up to retain the heat and let thoroughly dry. Of course remove the batteries! Also the electronics cleaner in a spray can will also do. Try to cover the hole in the sensor if at all possible, but it's not mandatory. The key is to make sure it's dry, before firing it up again.
I had one covered in BP residue and another that went to the bottom of a pond. after cleaning they both worked fine. One was a Perfectflite and the other a Missleworks.
Hope this helps.
PS you should not put the sensor in direct sunlight though, make sure it's on the bottom if alt. is in direct sunlight for any length of time