Do I need Marine Plywood for exposed porch

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I'm renovating an exposed porch area
Just removed the old T&G which was all sealed and very heavy painted.
I plan to replace with two sheets of plywood and a framing effect
None of it is structural just cladding

Confused on the different grades of Plywood

Examples from the Jewsons website,
ESSGEE SG MARINE PLYWOOD FSC 2440 X 1220 X 12MM £72 a sheet
Our Marine Plywood FSC is manufactured from high quality core and face veneers.The plywood complies with BS1088 standards vouching for its high quality. This Product is not intended for boat building.
Glue-line conforms to EN 314-2 Class 3 E1 and is suitable for use in exterior conditions subject to use with the correct edge sealant
Suitable for structural use in most moisture resistant and exterior applications but not recommended for boat-building
hardwood
Other products have similar specification

BRAZILIAN PINE STRUCTURAL PLYWOOD FSC 2440 X 1220MM X 12MM £31 a sheet
The plywood is bonded together using a E1 glue which is a low emission formaldehyde exterior quality glue.
Glue-line conforms to EN 314-2 Class 3 E1 and is suitable for use in exterior conditions subject to use with the correct edge sealant
Suitable for structural use in most moisture resistant and exterior applications.
pine softwood


LUMIN EUCALYPTUS STRUCTURAL PLYWOOD PEFC 2440 X 1220 X 12MM £33 a sheet
This PEFC Eucalyptus hardwood plywood is lightweight and strong and structurally certified to EN636-2S and can achieve EN636-3 with a suitable coating. It has an exterior bonding class 3.
Glue-line conforms to EN 314-2 Class 3 E1 and is suitable for use in exterior conditions subject to use with the correct edge sealant
Suitable for structural use in most moisture resistant and exterior applications.
hardwood


The costs are getting quite high saving >50%

Porch is exposed and open to elements , so gets a lot of wind and salty rain, as we are 0.5km from the coast

We will be painting all the board with probably a cuprinol garden colours
Also i will seal all the joints with CT1

We are getting on a bit, and so looking for minimum ongoing maintenance

looking the info here
https://elliotts.uk/plywood-classes-explained

appreciate your thoughts
 

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I am also on the coast. Even external ply will delaminate if water gets into the edges. I varnished sections for a bathroom floor and left the scrap and offcuts outside, they were ruined after a couple of weeks of rainy weather.

If you seal the surfaces with paint or varnish, both sides, give all cut edges multiple thorough coats (it will sink in) before assembly and protect the finished job with felt or something. The inside surface can be painted white if you want. Pay extra attention to upward facing surfaces exposed to rain, and low areas near the ground which will be damp and suffer splashing. Look at the edges before buying, poor quality ply will have visible gaps.

Marine ply is unnecessary expense.

I have had a boat made if marine ply and hardwood, it lasted well despite frequent immersion. I don't recall how the cut edges were treated before assembly, but I just painted the hull and varnished the top sides annually.
 
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Their is also cement fiber cladding and medite tricoya mdf.
 
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Just thought i should update, I went for MArine Ply in the end, had a few quotes for plastic cladding and prices ended up much to high.
I also had trouble finding WBP.
A few companies for the colour wife wanted would only provide a pallet > £1000
Thought I would add an image

Again thanks for all the help and advice
 

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