Hi Fred, it sounds like your starter is drying out on top because you've covered it a bit too loosely. The reason we say to cover the starter loosely isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded container and cause the lid to pop off. Try covering your container securely with plastic wrap (which will flex) and see if that prevents the skin from forming on top. If the temperature where you have your starter stored is higher than 80F, then it could be a bit too warm, but if it's in the 70-80F range then you should be fine. 

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