Best 1/4" Material For Doors For Outdoor Build?

[therailwayman]

A question for those who have done outdoor builds, what material did you use for the panels on your doors?

I'm going to use signboard for the back and sidewalls, but I plan on doing the "sandwich" technique (the 3/4" posts and rails with the 1/4" material in the middle) for my doors, and am pondering what to use for that 1/4" middle layer.  I haven't seen any 1/4" signboard and doubt it exists, but 1/4" plywood does exist.  I'm just wondering what the longevity of regular plywood versus plywood would be in this application.  I know priming and painting will help protect the wood, but if the marine-grade ply would last significantly longer, then I'd like to go that route, but if it's negligible, then I'll go with standard ply.

Just thought I would consult with the experts on this one.  Thanks, guys!

[Volpone]

Can't speak to marine ply, but my standard off the shelf plywood has been outdoors for 3 years and where it has been successfully protected it has held up wonderfully and where water has accumulated, I missed waterproofing, or the elements have worn it away it has rotted terribly.  My door is pretty much unscathed but the bottom of the wall next to it is pretty much rotted through and I wound up replacing the bit above the front topsign with some old Hardibacker I had laying around. 

Oh, one other thing I've learned from my build:  It is easier to trap water than you'd think.  I didn't bevel anything and not only do I have a fair amount of rot in the plywood at the base I've also found it behind the crossbars on the walls.  The rain hits the top of it and while some runs off the front, some manages to get in behind it where it just doesn't dry out. 

I also did false windows--just painted the plywood in those panels primer grey, stuck my fluorescent light diffuser to that, and then glued the window trim to that.  My reasoning is that it would be more waterproof and structurally stronger.  No way.  Uh-uh.  Somehow, water found its way behind the plastic "windows" and was trapped there next to the plywood where it worked its rotten magic on the wood.  This didn't happen on every window, but probably half of them or so. 

[galacticprobe]

One thing to keep in mind is that marine ply is very expensive. When I was active duty Coast Guard, for a while I helped out in the Boat Shop where they made the dayboards that mark the channels where the water isn't deep enough four buoys. The stuff is far more expensive than even good quality "normal" ply, and even then it wasn't immune to water rot.

Granted, we used half-inch thick ply, and these things were exposed to full weather (mostly getting rained on because for the most part they were high enough above the water to keep them out of the salt water). Still, the most we were able to get out of dayboards was about 3 years before they delaminated and had to be replaced. Taking that into account, I think volpone got some good mileage out of his non-marine ply, and it was certainly less damaging to his wallet. So get some regular ply which will save you some money, seal it well like volpone says, and I think you'll do fine without putting such a dent in your finances.

Dino.