Best coating for exposed plywood? Bed Liner? Full sun/heat exposure in a hot climate


Expedition Leader
I'm working up a roof tray that will fit within the perimeter of and be suspended from the rails of the Z-71 roof rack on my Suburban. The idea also spawns from earlier discussions here about shading the vehicle. So I wanted a solid tray. And hoo-boy is aluminum plate expensive when you start looking at 4'x8' sheets.

So I'm thinking to fab the tray from 1/2" plywood. I can seal / weatherproof it well enough, I've got the woodworking skills to do it all. But it's on a black vehicle, I want it to be black and 'blend in', not be particularly noticeable in daily use. So I got to thinking about stuff like wrinkle finish, epoxy paints, roll-on bedliner kits.

Anyone have any experience or advice / cautions, about coating the plywood?

Most of my ideas & executions are iterative. Lots of ideas, lots of designs, lots of Mk1, Mk2, Mk3 executions. Call it an experiment in progress, like everything else.

My original intent was to make it a single board. The overall dimensions are ~74"L x 44"W. It would be about a 1/2" shy of the inner face of the side rails of my roof rack and end near the rear cross bar and a few inches short of my sunroof opening. I figured to suspend it with flat metal straps / bar stock. Rigidity isn't too much of a concern for me, the Sub has so much interior room I haven't had to sling anything on the roof yet.
And the roof has a pretty good arch side-to-side. The rack 3-1/2" tall at the side rails, and 2" clearance at the centerline of the roof. And the roof ribs stick up about 1/2". So the tray bottom is going to sit ~1/4"-1/2" lower than the bottom edge of the side rails. And scarcely clear the two center ribs on the roof. So any real weight is going to get put on the roof skin pretty quickly.
The straps would cross from side to side, bent up at the ends, and bolted thru those ends into the side rails of the factory rack. T-bolts, fender washers on a stud, trapped with nuts, haven't figured that out yet.
The plywood would sit on the straps and I'd drill thru both wood and straps and tap the strap holes for some pan-head machine screws to hold the wood to the straps.
I also thought to make some oblong holes in the plywood, oriented perpendicular to the strapping, so that tie downs could be hooked to the metal straps to tie stuff down to the tray.

Then I thought about it a bit more. I've been gearing up for both HAM radio vehicle-mobile ops as well as looking into CERT training. And planning some southwest desert trips. I used to hang around out there all the time, 30yrs ago. Anyway, it occurred to me to make those oblong holes hand-sized, like handles, the easier to handle installing or removing the tray. Then that sort of reminded me of back boards and stretchers

So that's when I figured to make the tray in two sections. Same general config, but split down the middle, with a bit of rounding of the (additional) corners. And move the hand holes closer to the periphery of the sections. A couple minutes with a philips screwdriver would be all it takes to have a stretcher board.

I've got a lot of unfinished projects / simultaneous crap going on. This is another. But it might get done much sooner as it's easier and much cheaper to complete. Trying to figure out what sort of durable finish will bond best to plywood. I might go with a slate gray color, but I'm leaning towards black. Definitely won't be anything light. I don't want it to be noticeable at a glance. Maybe I make the bottom and edges / handholds black and topcoat it a lighter color.

Design suggestions are welcome, those scribbles above are all I have figured out at this point.

I'm also wanting to put a 40-42" LED light bar and air dam up top, but those will be mounted to the front shoes of the Z-71 rack, right behind the sunroof. The air dam will cover / hide the lights and I can reach up thru the sunroof to pivot it up to unmask the lights. And the light bar will have a bottom glare shield to block the light from shining thru the sunroof.
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If ply is fir you should use the best grade you can find and coat the exposed side with 4oz or 6oz fiberglass & epoxy using the advice available on line from from West System or System Three. The underside can just be epoxy sealed. If using marine ply you could skip the fiberglass. The way fir veneers are rotary cut makes them prone to surface checking - that's the reason for the glass. With your proposed multiple cutouts you'll have lots of ply edge grain that needs sealing & multiple epoxy coats is the only way to do it long term. Water ingress into the ply edges is typically the failure point.
It looks like you plan to leave the edges bare ply - not covered by a trim piece - I'd suggest marine ply or baltic birch before fir ply as the edges typically don't have voids and finish nicely.

The black color is problematic as desert sun heat would be close to the limits of epoxy - if the top could be white or light grey the epoxy would be happier. If you stick with black a layer of fiberglass would help make it last. These materials can be finished as fine as you like if that's what wanted. I'd still coat with epoxy if using bedliner which will look like dirty bedliner after awhile...

Excellent idea - just make sure your tie downs work...I use hose clamps threaded through slots in the ply and around the roof rack bar.Moe


Regular exterior latex paint in black will work perfectly. It is designed to expand and contract with the wood. A quart is more than enough and you can repaint it in a few years if you feel the need.

Exterior latex paint is rock-hard once it cures. It is probably a good idea to prime the wood first.

I just pour the paint directly on the plywood and then roll it out with a foam roller. Leave it in the sun for a couple days for the paint to cure all the way through. A foam roller will give you a pebbly texture similar to bedliner.

Easy peasy and inexpensive.

Postscript: don't get hung up trying to get the wood completely impermeable to moisture. Wood breathes and your roof rack is not a boat. Exterior latex paint is designed to topcoat wood and to be exposed to the elements. Some of the products mentioned could eventually crack from extremes of temperature as the wood does what wood does. Once they crack, moisture gets inside and the finish can lift.


Rendezvous Conspirator
Spar urethane. I'm not sure how well bed liner will adhere to bare wood.
I coated the inside of a battery/tools/etc. box with brush-on bed liner, and it worked fantastically well. Adhered very well and was very hard wearing. Have since broken that box down, planning to recycle the wood into other things, but it turns out that bed liner is never coming off unless I use a surface planer, so I've just cut it up for jack cribbing now.


New member
Exterior Latex

I had built a 52" x 86" all wooden deck on the top of my camper shell. Big box purchased 1/2" for the deck surface. I just wanted it to make it through one road trip season and look at fabricating something better for the following season.

Anyway, I made no serious attempt at proper treatment or appropriate efforts to provide long-term protection for the surface. I just purchased the least expensive exterior latex paint and applied with no primer. Rolled and/or brushed two coats on.

Three years later, it remains on my truck (exposed to full-time UV) and to my amazement, it has performed a good job, given no real surface preparation.

Whatever level of protection you want, shouldn't be a problem with an exterior latex, but no doubt, there might be some good arguments for the marine coatings.


Polyester resin thinned 3 to 1 acetone makes a good primer for plywood thinned it will penetrate well and has served me well in the past.


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I highly recommend truck bed liner. I made a plywood shelf for my rear bumper that I had professionally coated in truck bed liner. It was a step and a shelf to support a propane tank and folding ladder. It withstood all kinds of abuse, sun and water with no signs of deterioration or fading.

Rear Bumper Shelf.jpg


New member
I built a bed rack from 3/4" exterior grade plywood. I cut some large oval holes in for both weight reduction and access to tie down points. I used an entire quart of Herculiner with a single coat on the underside and as many as I could get from the can on the top side and the edges. It's mostly on the truck in the winter and doesn't come off until the spring. The herculiner has held up great.


Expedition Leader
Thanks for all the great and varied replies. I'm still considering. I've used spar varnish in the past on some backyard structures that are horizontal / exposed to the sun and it didn't last two full years in our sun / heat. LIkewise my neighbor has been plagued with re-varnishing the wood bed planks in his '56 Ford pickup. Lquers and varnishes just don't seem to take our exposure anymore. Maybe it's CA's no-VOC edicts.

I've done the bed of my pickup with a Herculiner roll-on kit, but I felt it's a bit too gritty / abrasive. Great for traction surfaces, probably a good thing for a roof tray, but I don't know.

The external latex appeals to be greatly for ease of use and cleanup and application and I'm glad to hear it bonds well. I should have considered that right off, as I did a fuel and water cabinet (like a paint locker) on the south side of our house, summer of 2011, and it's skinned with T-111 siding. KILZ primer, good Behr exterior latex. It's held up very well. I painted the whole house that summer as well, same way, and primed the trim on the south and west faces. All still good. So that's a big contender.

What I'll probably do is paint the handle cutouts and bottom sides black and the top surface I'll have to think about some more. It would make more weather / sun sense if it isn't black. Lighter the better for longevity, I would think. Since it's a false roof and not in contact with the roof skin, I'm not too worried about heat transfer across the air gap. Shading 75-80% of the roof is a big enough improvement. It's an all black suburban in sunny SoCal so I'm pretty much screwed anyway.
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nick disjunkt

Can you not get buffalo board type panels in the US? Usually it's birch plywood with a textured phenolic film on one site and smooth on the other. Seal the edges and it's last for ever. It's used as flooring on trucks and trailers in the UK but it might have a different name in the US.

I used it all over the roof of my truck and it's lasting well. I used 1/2 inch as the spans were pretty small.


Ducky's Dad

Maybe it's CA's no-VOC edicts.
It absolutely is the ARB and SCAQMD regs. You cannot get real or even decent spar var in Los Angeles. Most of the "spar var" at the big box stores is water-based polyurethane masquerading as spar var. You can import your own from AZ, or you may be able to score some at the harbor in San Diego. There are a few places that supply the Navy that may have something suitable. I have had good luck at the Sherwin Williams Pro Line store at the harbor. San Diego has slightly different regs than the rest of So Cal. You may be able to have decent spar var shipped in from out of state, but only in small quantities. You may also be able to modify the crap that is sold here (if you can find some solvent based stuff) with turps, boiled linseed oil, real mineral spirits, and Japan drier. It will take some experimenting to get it right. One of the characteristics of real spar var is that it relatively soft and flexible to accommodate the wood's movement with moisture changes, so it scratches easily and will need to be refreshed every couple of years. I would not use it for what you are trying to do.