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Welcome to the Bunnings Workshop community @Tala. It's brilliant to have you join us, and many thanks for your question on sealing MDF.
Could you tell us a little more about your need for the MDF to be moisture/water resistant? Typically, your mosaic won't see significant moisture or water if displayed indoors. Do you intend to hand the mosaic in a bathroom or high humidity environment?
If the mosaic is displayed in your home, coating the MDF's rear side with varnish will seal it adequately. Or, an enamel paint will also do an excellent job of preventing water penetration. You might even like to use Boyle Clear Matt Spray Sealer if it's more convenient.
The front is already sealed with your white base, glue, grout and tiles. It would be tough for moisture to penetrate the front of a mosaic unless it's kept outside in the weather. For an additional layer of protection, you could apply Davco Sanitized Tile and Grout Sealer. This will prevent any moisture from penetrating the surface of the mosaic.
Please let me know if you have further questions.
Mitchell
The suggestions I've provided above would be suitable for most scenarios inside a home. If it's going right next to a shower stall, we might need to rethink. If that were the case, I'd recommend you use a board more suited to moisture exposure, like marine plywood. MDF is highly susceptible to degradation in moist environments.
There are plenty of acrylic water-based enamel paints these days, like the one I linked above. You'll be able to cover them with your acrylic paints.
Mitchell
