How to make MDF waterproof for mosaics outside?

[kerry65]

I do mosaics -the reason we can’t use mosaics on wood outside is because apparently wood expands and contracts popping off the tiles. I can make shapes out of mdf. Does mdf expand and contract. How do I make the back of the mosaic waterproof. The front will be glass and epoxy grout so that’s fine. I really want to make weatherproof outside shapes to mosaic on.

[TerryN]

@kerry65 

Hello Kerry,I just was having another look at the shapes you use with your mosaics, and was thinking if you use the same shapes regularly that you might be able to make concrete moulds for them just by using the area of the shaped piece that you cut out.(so instead of using the timber you cut out,you would fill the shaped gap left with concrete ) I can't see how deep they are but they seem to be around 20 mm or so?It was just a thought,and wouldn't be hard to do providing the pieces you need are flat I suppose,as they appear to be in the snap.It would then just be a case of getting the right thickness of timber to cut the shape from to give you the required depth.That would mean that your tiles would be going over concrete and that would be a suitable substrate to work on,and you would be able to use the same mould over and over.

Regards Terry.

[kerry65]

That’s a great idea now just got to work out how to do that .