Heat shrink can be used over open coils if you want.
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FWIW, I leave the coil open so that it stays clear of ice in the winter. I prefer an open coil over a closed one for a couple of reasons. One it's more robust, wound coils are usually delicate and the magnet wire thin. Second, being closed they usually end up leaking and that causes corrosion and sometimes a freeze/thaw cycle ruins them.
