From my experience, the typical 3D print is not watertight. At least mine are not. It may be my settings, my printer or the fact that I print almost only PLA. I tested several times and usually water finds its way somewhere between the layers.
Question is: Does a micro SD card need water protection at all? The cards look waterproof to me. Of course, they need to be dry before voltage is applied.
This seems to be true for some cards at least:http://kb.sandisk.com/app/answers/detail/a_id/4687/~/sandisk-cards-environmental-tolerance-(waterproof,-temperature,-magnetic-and