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How to make MDF waterproof for mosaics outside?

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.

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MitchellMc

Hi @kerry65,

MDF would be considered even less moisture tolerant than timber. They'll both expand when saturated with water, but whereas the timber will dry out and return to a similar original size, the MDF won't. Once the fibres of MDF contact moisture, they expand like a sponge and then disintegrate. So, you might be better trying a medium that is already moisture resistant and then painting the exposed back of the board with an exterior varnish. You can consider using marine plywood, moisture-resistant particleboard, moisture-resistant MDF. Perhaps things like Villaboard and acrylic or polycarbonate sheet could be used. I suspect you'd need to scratch the plastic up heavily, so the glue has something to grip to.

Did you intend to have these mosaics out in the rain? I've had a mosaic on standard plywood outside but undercover for several years without issue. The trick to having them in the weather will be encapsulating the back panel so moisture can't enter.

Let me mention some of our mosaicing enthusiasts @Julieg9426, @maryg, @TerryN, @Mammasalt and @Ezzy to see if they have suggestions for a suitable medium for exterior mosaics.

Mitchell

kerry65

https://www.workshop.bunnings.com.au/discussion/comment/17776#Comment_17776

Thanks Mitchell I may have to go with a fibreglass. In the mosaic community we use marmox board or cement sheet for outside shapes but both don’t cut easily on my scroll saw. Plus I wanted to keep costs down . Apparently marine ply also moves contracts popping off tiles. However I’m not sure if this is proven. The other wood/woods you mention do they contract in the weather they will be outside in the rain hanging vertical.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/71532#Comment_71532

Hi @kerry65

Among the woods that my colleague @MitchellMc mentioned, only the marine plywood would have a significant amount of movement. The rest of the timber pieces mentioned would have very minimal movement. What kind of tile adhesive have you been using for your mosaics? One of the best ones I can suggest for your mosaics is Dunlop 20kg Rubber Modified Tile Adhesive It actually contains recycled rubber and is extremely tolerant of tile and substrate movement.

If you need further assistance, please let us know.

Eric

MikeTNZ

Personally kerry65,

I would use something like H3.2 grade plywood, it is made for wet areas.

MDF has a shocking attribute, no matter how well you seal it, it will ALWAYS expand when moisture gets in.

And as much as you try to seal it, moisture will always penetrate MDF.

It's fine in a kitchen where it doesn't get wet or stay wet, but the plusses in price outweigh the minusses.

Do it properly, do it once.

Let us know how you get on, kerry65, I love moisaics, I'd like to see how this looks.

Thanks,

MikeTNZ

kerry65

https://www.workshop.bunnings.com.au/discussion/comment/99046#Comment_99046

Thankyou Eric.

I have been using soudal t-rex power crystal.

I will try your suggestions. Thankyou I like the soudal because sometimes I put mirror in my work and it doesn’t affect the backing. But I could use both.

kerry65

https://www.workshop.bunnings.com.au/discussion/comment/17783#Comment_17783

Thankyou Mike.

TerryN

Hello Kerry,I can't really answer your query as I've only done a couple of mosaics exposed to the weather,but will tell you about my experience with substrates in general as Mitchell tagged me in relation to your comment.I usually use pieces of yellow tongue flooring for most of my mosaics as they are sitting in a covered pergola area and I have access to lots of them on the building sites I work on.They are also easy to attach a wooden frame to and I also do that on most of mine.Even though they are not in the weather I usually put a couple of coats of Bondcrete on them and also a waterproofing membrane,which maybe isn't needed but it's a fairly quick process and I usually have the products handy anyway.I have used compressed fibro flooring for a couple and used the wet area waterproofing membrane on them as well.They are out in the weather and seem to be ok after a few years.I'm about to start one on a large piece of villaboard cement wall sheeting,which is left over from renovating my daughter's two bathrooms (along with a heap of tile off cuts,etc,which will be handy.I haven't done one on the thinner cement sheeting yet but have heard of other's doing it on it.I'll include a couple of my pieces for your interest Kerry.Regards Terry.

kerry65

https://www.workshop.bunnings.com.au/discussion/comment/17794#Comment_17794

Thanks-love your mosaics. I sell my mosaics so want them to last outside. Don’t like marmox board or shower board (forgot name) love pine and water resistant pine but apparently any wood outside is a no no in the mosaic community. The tiles apparently pop off in time However with all new products and glues I’m sure it can work.I’ve found the advice in this thread very helpful so will try a different glue and the wood suggested in this thread.

TerryN

https://www.workshop.bunnings.com.au/discussion/comment/71543#Comment_71543

Hello Kerry,Now I can see what you mean about the shapes you need to create.They look terrific.I know what they say about using timber and timber composites outside as I have a friend who does heaps of mosaics, and she is up with all the advice that comes from the craft people,but my experience is in the building industry,and the waterproofing products available are made for that, so I can't see why you can't waterproof marine ply etc.We use marine ply over and over again in formwork for concrete and it lasts for years.I might have to do a bit of experimenting and see how it goes.Again,your work looks great.Regards Terry.

TerryN

https://www.workshop.bunnings.com.au/discussion/comment/71543#Comment_71543

@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

https://www.workshop.bunnings.com.au/discussion/comment/99056#Comment_99056

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

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