Thread: Do I need to seal/varnish Marine Ply

Wish my leaks were in the shed (they're in the house actually)

My understanding is that marine ply is fundamentally different in the type of glue used, as well as generally being a little better re voids etc. But if water is still lurking, cover it!! Marine varnish would be best, easier than poly to apply. Trouble is it is expensive. I use Bondall from Bunnings and i think it's about $35 for a litre, which has lasted a long time so far.

Definitely have to seal it. Any waterproof coating will do (varnish, floor polish or paint)

Marine ply uses exactly the same glue as regular structural grade ply (phenol formaldehyde), the only real differences are:

- Marine ply is A or B grade throughout, so that there are no internal voids, whereas structural ply may have voids and is of lower visual quality.
- Marine ply is not made to comply with building codes, structural ply is.

Taken from Carter Holt Harvey http://www.chhwoodproducts.com.au/us...5/file/568.pdf

Thanks Elan, I had always thought that marine ply was made with a truly waterproof glue, but there you go ....!

"Marine Ply is not made to comply with building codes, structural ply is..."

Ya, sort of. "Proper" Marine Ply - the stuff thats made solely from face ply with no voids, crosslaid sheeting, marine grade glue, marine rated wood species etc etc - is made to sheath hulls. Really really good Marine Ply will be certified by a Classification Society: Lloyds, DNV, BVS etc etc.

CHH are doing a bit of clever marketing there: Structural Ply meets the minimum requirements of building codes. The bit they dont say is that marine plys far exceed them. And having just purchased a pack of CHH ply myself I can tell ya that they really are aiming for the minimum requirement. It was without a doubt the worst single pack of ply I've ever purchased, nails disappearing into voids everywhere during use... and would you believe that they'd even managed to cut the sheets out of square? The whole bloody 40 of them??? Theres no requirement for square under code I guess. Me not happy with CHH this month.


Mostly what is sold as "Marine Ply" in Australia is actually exterior grade "waterproof glued" ply. If you go get CDX it may or may not be waterproof glued, there is no requiremnt that they do so so different manufacturers may use different glue. Real Marine ply is made with minimum F14 timber, that is free of knots or voids, and that is durable when exposed to water. Mostly the "marine ply"you'll find here is made of non durable hardwood ex SE Asia, and wont meet marine spec either.

Thanks for the replies.

Bendigo Bob, sorry about your leaks.
Like others I thought marine ply used a (more) waterproof glue. It does have a more solid feel and weight to it.
Looks like I'll have to varnish them to be safe. The Bondall is cheaper than most of the other brands exterior and marine varnishes.
The marine ply, though more expensive, was cut to exactly the size I need for the shelves, 1220 x 610. All the other plys at Bunnings are 1200, 2400, 600, etc. I'd have to have it cut and end up with lots of extra panels, which I may never use.

I build boats out of marine ply. It is far better than structural ply. But is hard to find. Cheap mis tinted paint from Bunnings is enough for a good while. Best thing to do is paint it to seal and fix the roof