Mimi, Q-What is "naval plywood" - I am guessing something that can withstand water/rain?

I don't have an answer to your question on sealing as I don't do the mosaics, but you just hold on cause several of the members here do and I'm positive you'll get the answer you're looking for.

Hi Mimi,
I always use sanded grout because my gaps are so big and most of my things are for outdoors. Read the label on the sealer to make sure you can use it on unsanded grout and make sure its for outdoor use. (resists water)
I am not a professional, you can ask at the hardware store for the best sealer they have for the job.
Can't wait to see the photos.
Lori

Have you visited the stained glass and mosaics forum? They can help you with your question. Here is a link:

Here is a link that might be useful: stained glass and mosaics