Hi Susi, to avoid developing a crusty top on your starter it's important to cover the starter container securely, either with plastic wrap or a lid. The reason we say to "cover 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. As long as you have room above the starter level for gases to accumulate, and open the jar now and then to vent the gases, it's fine to place a lid on your container. Plastic wrap will flex as gases accumulate, and will work fine too. I suspect that keeping your starter moist throughout will help with a more even bubbling, since the bubbles tend to rise to the surface over time, and this movement may be inhibited by the crust that is forming. 

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