Durability Issues with an Outdoor Wood Mosaic

Advice on a tough challenge: finishing a multi-piece, panel-backed, wood mosaic, so it won't tear itself apart from moisture-related wood movement. March 29, 2008

I recently completed a project for myself and need some advice. It is an outdoor mosaic display made from poplar and backed with 1/4" exterior luan. It was stained with Minwax and finished with 4 coats of Helmsman spar urethane. The pieces are like a puzzle and the backing holds it together. The pieces were both nailed and glued to the backing. As I stated, it is an outdoor project and the weather has caused it to twist and separate from the backing. I have several more to make and I am looking for advice on sealing the pieces and making them more weatherproof. I have considered using Envirotex Lite and possibly changing the back to 1/2" treated plywood, but I am not certain if this will help.

Forum Responses
(Finishing Forum)
From contributor E:
I would not use poplar. It doesn't do well outside, in my experience. I'd look at white oak or cedar and see what happens.

From contributor D:
Use Extira MDF.

From contributor M:
Coat as well the back of the piece and all edges.

From contributor J:
You're asking for trouble when you glue pieces of solid wood larger than a couple inches across to any engineered substrate. Regardless of the finish you use, the solid wood pieces will absorb moisture and expand while the backer stays roughly the same size. No glue in the world can hold the two together. If you're dealing with solid wood chunks of any size, you need to allow them to grow a little without doing damage, probably by using screws from the backside. I'd also try to minimize damage done by water that gets between the backer and the puzzle pieces; finish all sides of all parts before assembly. Lastly, I'd want to minimize the punishment they see by installing these under some sort of overhanging roof - full exposure to sun and weather is brutal.

From contributor A:
A couple of things to consider:
1. Poplar is not an exterior wood. It rots quickly when used for exterior carpentry projects.
2. I've never come across exterior grade luan plywood. 1/4" Okoume marine ply is $90 a sheet.
3. 1/4" plywood is not stable lying on a workbench. Coating and weighting it on one side is asking for serious warping.
4. Minwax stains do not perform well in direct sunlight. Helmsman is about the cheapest spar varnish available. Decent stuff is $60/gallon, marine is $100+/gallon.
5. Envirotex is an epoxy bar topcoat. Epoxies have no UV protection. They require 3 coats of varnish over them.
6. 1/2" (pressure) treated plywood is not meant for much more than sheathing something near the ground. It is totally unstable off the shelf.

I would need to see a picture before making any suggestions on proper construction. The finishing is the easy part; the fabrication is delicate.

From contributor C:
In your design, you need to allow for wood movement of the solid wood pieces, and for uniform moisture absorption and dissipation. To accommodate wood movement, I suggest that you don't glue the pieces to the backing, and that you allow some distance between the individual pieces to permit expansion across the grain. Screwing through the backing into the center of each piece will allow for movement. Depending on the sizes of the individual pieces, 1/32" to 1/16" spacing should be adequate - if they're fairly small. The correct spacing to use depends on the temperature and humidity during fabrication. There is no finish that will absolutely seal wood against moisture. To allow for the most uniform moisture absorption and dissipation, stain and finish all sides of each piece exactly the same. The stain and finish you are using will work well outdoors. Realize, though, that all clear exterior finishes, depending on exposure to sun and rain, will need refinishing, or at least sanding and recoating every several years. Additionally, I don't think that 1/4" plywood is the best choice for a backing material in your application. My choice would be 1/2" or 3/4" Extira, a highly water resistant MDF which is quite stable.

From contributor B:
Composite decking material comes in different colors.

From contributor U:
Good advice has been given in above posts. Extira has been in use for a while now. When it came out I didn't believe it, so I sunk a piece in the ground about a foot, left it for a year, and no swelling whatsoever. A company I buy exterior wood shutters from uses it for flat panels.

You can get exterior luan (meranti) with phenolic glue. It will have the letter p before the grade (usually bbcc). You can also get pressure treated marine ply. Both of these would not be the best choice for substrate. Still you have to deal with the movement that will happen with the mosaic pieces. There is no finish that is a complete vapor and liquid barrier that will stop moisture from coming and going through the solid wood.

From contributor Y:
Best exterior wood? Teak. Best above the waterline exterior teak finish? Marine grade polyurethane as tie coat, then polyester for build. Good enough for 40 million dollar yachts - good enough for your project.