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Assuming you treat both the marine plywood and concrete plywood with an edge sealer.

What would last longer out in the rain...marine plywood painted with exterior paint....or concrete plywood which comes with epoxy already applied.

I know the most durable would be the marine plywood with epoxy but I hate applying epoxy. Thank you.

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  • Epoxy is susceptible to damage by sunlight, so if you're using it in a project exposed to the sun you'll want to paint it or otherwise treat it to block the UV. And marine plywood might be more than you need... there are exterior-rated plywoods that might work for you.
    – Caleb
    Sep 26, 2022 at 15:18
  • Last summer, I covered my shed walls with T1-11 siding. It is, effectively, plywood with some "wood grain" texture on one side. I applied 2 coats of paint. I expect, with a fresh coat of paint every 3-5 years, that it will last many decades. The siding itself has no waterproofing preapplied (that I'm aware of), though it may use a waterproof glue/resin during its assembly at the factory. How long do you need this to last in the rain and how much maintenance are you willing to do to get there? Nothing will last forever without any maintenance...
    – FreeMan
    Oct 18, 2022 at 16:07

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I don't think you can get anything like a definitive answer to this since plywood is now so notorious for how much it varies within a given class. As a result, even if you managed to find an existing head-to-head comparison that looked thorough (best of luck with that!) it may not tell you anything that is actually directly applicable to your situation.

If what you're doing is really critical probably the best advice is to run some tests of your own, assuming you have the time.

If you don't have the lead time then "you pays your money and you takes your chance" as the saying goes. But FWIW consider that concrete-form plywood, although expected to withstand getting wet regularly, is not actually made for continuous exposure to the elements.

For what it's worth, assuming you buy a reliable brand I doubt you'll be disappointed in the performance of the marine-grade if treated right; this seems like it's right in its wheelhouse :-)

With edges sealed well with CPES (probably the best sealant option, assuming no UV exposure) I think you're likely to get the best long-term durability. If any edges are exposed to UV they should be further coated with something to protect the epoxy.

Note that upkeep will be necessary if a long service life is your intention, this means regular refreshing or renewal of the chosen topcoat (possibly annual in the case of transparent finishes).

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