Marine or Exterior plywood for Jonboat deck

[Jonboat2Bassboat]

I am extending the front and rear decks on an aluminium Jonboat and upgrading the aluminium sheet metal floor. These will be surfaced with marine carpeted plywood panels and screwed onto the boat as a unit with SS screws.

Been reading the repair forums here and seems there is no clear answer on using marine or exterior plywood on aluminium boats. Hopefully a few answers on what I have read will help me decide.

Have read marine plywood is treated with copper like PT plywood and should not be used on aluminium. True?

If no, marine plywood can be used but needs to be sealed. If yes, use exterior plywood but it too needs to be sealed.

If both types of plywood need to be sealed, what to seal with? Spar varnish, Polyurethane or some sort of marine epoxy?
One guy said only seal the bottom and edges of the plywood as the adhesive will stick better to the unsealed plywood and, with the marine carpeting, will provide a waterproof seal for the top.

If sealing is required, what carpet adhesive should be used? Have found several marine carpet adhesives from Henry's 663, DAP / Weldwood Contact Cement and ********* carpet adhesive but none that work with sealed plywood.

Any thoughts?

 

[minuteman62-64]

Re: Marine or Exterior plywood for Jonboat deck

My understanding is that marine and exterior ply both use the same glue. The difference is that the marine plywood doesn't have interior voids - therefore it is stronger, particularly when bent or stressed over a framework.

Never heard about marine ply being treated with copper. The glue of any plywood, when it gets wet, can leach chemicals that will corrode/make pin holes in aluminum in an area with limited ventilation. For example the plywood used for transom reinforcement in an aluminum boat can cause pin holes in the aluminum skin if it isn't sealed.

 

[kfa4303]

Re: Marine or Exterior plywood for Jonboat deck

+1. Marine grade is just void free and made with high quality waterproof glues, but isn't special in any other way except for the price. For your particular project, I would suggest regular old non-PT, exterior grade plywood (1/2"-3/4" up to you) and seal it with several coats of Spar Urethane, then fasten it to the hull using SS hardware dipped in 3M 5200. You can get everything you need a Lowes/HD in one trip, if you plan ahead carefully. You can then paint over the urethane, or leave it as is. They even sell aluminum angle, tubing and flat stock all of which are handy for making bracing and ribs. You can even use the pink/blue/green closed-cell insulation foam board as below deck emergency flotation. Good luck. Keep us posted.