Marine ply or Birch ply for internal panels?

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Babylon

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For non-wetted internal areas is it really necessary to use marine grade ply? I'm partially replacing engine-bay side panels and have lots of offcuts of furniture-quality Birch-ply (not the cheap shuttering-ply that builders use), which I'll be painting with bilge-paint. I assume that this will be more than adequate?
 
Babylon said:
For non-wetted internal areas is it really necessary to use marine grade ply? I'm partially replacing engine-bay side panels and have lots of offcuts of furniture-quality Birch-ply (not the cheap shuttering-ply that builders use), which I'll be painting with bilge-paint. I assume that this will be more than adequate?
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It isn't just a matter of whether it will get wet. Ordinary plywood will not last long in a damp, poorly ventilated area.

I only use Robbins Elite or Super-Elite in my boat. It is expensive but seems to be very good quality with many laminations and an attractive face veneer (although that doesn't matter if you are going to paint it). Compared with the amount of time you will spend making a good job of these panels I don't think the cost of decent ply will make much difference, especially as you will probably only need a half-sheet.

But I'm not telling you what you should do - just what I would do
 
I've used birch ply. It is normally WBP i.e. the glue used to bond the veneers can withstand boiling water, however the veneers themselves will swell if they get penetrated by water. You therefore need to see that it's well primed and painted to prevent moisture ingress - especially the end grain. Then it will be fine. Good quality birch ply is extremely strong, it was the type of ply used when wood was a structural component of aircraft. The Mosquito was made mainly out of birch ply, much of it as a ply/balsa/ply sandwich.
 
Marine ply is only necessary for exterior areas. Good quality WBP exterior uses the same glues as marine and often the same timber. The difference is that there are no voids in marine and there may be some in exterior. WBP stands for "Waterproof Bonded Ply".
By the way my internal fit-out used all WBP and is still good 26 years later!!!
 
I would say birch would be fine, however before painting, I would coat the whole lot, especially the end grains, with thinned epoxy. Then you will have no worries atall.
I even do this with marine ply, the end grains if not all of it, anyway.
 
I have a 4 wheel 'builders' trailer, imported as a kit from Estonia or somewhere like that, now 8 years old, used to carry bricks, blocks, rubble, ballast etc. from time to time as well as moving timber and boat stuff. It has never been under cover, the floor is a single sheet of bare birch plywood and while well scuffed is good for another 18 years of similar treatment.
Years ago we specified birch ply for the ceiling panels of multipurpose community/sports halls with just a light stain finish, we wanted acoustic performance as well as impact resistance as the room was used for football training,drama, concerts etc. Halls like this are a real test of condensation resistance as they can be unheated half the time with cold surfaces and then have several hundred people in them for a few hours, and people produce an awful lot of moisture, the ceiling performed very well.
 
Super info. Thanks for all the feedback.

Will seal the birch ply with epoxy (also the sawn edges of the remaining original marine ply that I had to hack out to access hidden oil-contaminated areas).

All these new panels will be screwed on and sealed with sikaflex, effectively turning them into inspection-hatches for the future.

Cheers

Babs
 
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