Making exterior plywood waterproof?

[Ruggers]

I'm looking for any suggestions and products that I can buy to paint or lay onto 8x4 ft sheets of ply wood to make it waterproof. the sheets will be used for the back outer walls of a summerhouse which is fitted in the corner of a garden and not much gap at all to maintain it so I want it to last.

one option was to fibreglass them with the same UV resistant product I use on the roof but this seems expensive. I've 6 sheets to seal.

The two sides are facing away from the elements so 9/10 won't take much weather.

 

[Xenon]

Just affix a layer of felt to it.

 

[catlad]

I am planning to replace an old rotting shed in the bottom corner of the garden soon, and I will be using box profile steel profile sheets on the sides that I will have no access to when the shed is finished.

 

[Beebopper]

I've got a build in a similar position - have used correx sheets pinned to the ply and sealed with waterproof gaffer tape on any joins, this is for any non-visible parts of the build. Any visible parts have been clad with upvc cladding.

 

[Ruggers]

When you say felt do you mean nail on or the torch on mineral felt?

I did think about the UV resistant clear plastic not sure of its name, its like round corrugated held on with screws and water tight dome caps then you can push in foam inserts to blank off the gaps at the ends.

I can just and so slide up to get screws in to fasten the panels or screw them from the inside.