My desired use of plywood (exterior) what "waterproofing?"

[qwerty11]

I am wanting to cover my roof/sides of my camper with untreated plywood. If all edges of the plywood are not exposed to water because they are covered with trim/flashing, and the face of the ply is primed and painted with RustOleum epoxy paint, do I need to do the 75% mineral spirit/ 25% poly sealant? Will the ply hold-up?

What type of caulk should I use for the flashing/trim seams? Silicone, latex, PL adhesive???

How does everyone fill plywood seams on exterior joints? I was thinking about leaving a coins worth spacing and just filling it with PL adhesive. Should that work?

[TPMcGinty]

I used Bondo Wood Filler. Easier to sand than adhesives.

[les45]

I used 4" fiberglass tape and Aero Marine epoxy on all joints. This adds strength to the joint and lessens the chance of cracking of filler materials and paint. The tape wrapped around the corners and also covered the spar screw holes (see pic). I used Smith's Fill-it (compatible with epoxy) for all small cracks, blemishes, screw holes, staple holes, etc.
I also added vinyl insert aluminum edge trim on all square edges after final painting.

[Oldragbaggers]

Great job Les, really smooth.

[les45]

I am wanting to cover my roof/sides of my camper with untreated plywood. If all edges of the plywood are not exposed to water because they are covered with trim/flashing, and the face of the ply is primed and painted with RustOleum epoxy paint, do I need to do the 75% mineral spirit/ 25% poly sealant? Will the ply hold-up?

What type of caulk should I use for the flashing/trim seams? Silicone, latex, PL adhesive???

How does everyone fill plywood seams on exterior joints? I was thinking about leaving a coins worth spacing and just filling it with PL adhesive. Should that work?

It appears you have changed your original question and added some more. I'm not sure what you mean by the 75/25 question but I will try to cover all your concerns:
Even if you use edge trim, you still need to seal all joints to make them watertight. What I mentioned in my first response above about fiberglass tape and epoxy is how I did it.
I used untreated plywood for all the exterior skins on my weekender and this is how I treated it:
1. Coated all exterior surfaces with CPES. I applied four coats just to use up the entire batch. Some folks will say that one coat is enough. CPES is very thin and goes a long way. I also applied the CPES to all other exposed edges like door openings, hatch, and tailgate.
2. Filled all screw holes, staple holes, cracks and blemishes with Smith's Fill-it which is compatible with epoxy coatings and fairly easy to sand.
3. Applied 4" fiberglass tape and Aero Marine epoxy on all joints.
4. After several coats of epoxy and fillers and all surfaces sanded smooth, I applied two coats of Zinsser oil based primer with 4" foam roller.
5. After more sanding and more filling of any remaining blemishes, I used Rustoleum oil based enamel for a top coat applying three coats with 4" foam roller. The last coat was thinned by 25% with mineral spirits to get the smooth, shiny finish (that may be where you got the 75/25 reference). Note that this Rustoleum is not an epoxy paint.

For my metal edge trim, I used butyl tape under the full length of the trim. Also, I added a drop of silicone in each screw hole before inserting the screw.

As you can tell by the above, your entire exterior must be as watertight as possible or water will get in through any small hole or crack, even under your trim. What I did is just one way of doing it. Others have coated the entire skin surface with epoxy (expensive) while others have used tractor or boat paints. You just need to do your homework on which type of system you want to use and how much you want to pay for it.
Good luck
Larry S.

PS - if you have more questions concerning the reponses in this thread, it would keep it better organized if you hit the "quote" response in the upper right corner if you have a specific followup question or hit "post reply" in the bottom left.

[qwerty11]

I've been reading quite a bit about using uncovered ply for the exterior and I'm about afraid to use it. Seems like a may be better to go back to my original idea, FRP glued to 1/4" ply.

[TPMcGinty]

I've been reading quite a bit about using uncovered ply for the exterior and I'm about afraid to use it. Seems like a may be better to go back to my original idea, FRP glued to 1/4" ply.

I don't know if I'd glue it. I think some people have had problems with thermal expansion of FRP. Check it out before doing that.

[les45]

I've been reading quite a bit about using uncovered ply for the exterior and I'm about afraid to use it. Seems like a may be better to go back to my original idea, FRP glued to 1/4" ply.

I don't know if I'd glue it. I think some people have had problems with thermal expansion of FRP. Check it out before doing that.

Also check to see if you are using an exterior grade of FRP. Interior grades will not hold up to UV exposure. You will still have to seal the joints very carefully even if you use edge trim.

[working on it]

I went the route of poly mix (base coat)/ tractor paint (top coat) over plywood. I even used Silo paint (aluminum heavy-duty) for the front, roof, and sides - for weather resistance and reflectivity. All plywood edges (exposed plys) were either vertically oriented, shielded by a rain deflector (hatch opening cover), or coated with PL adhesive (waterproof when dry). then they were painted over. The holes in the roof (where 1/4-20 bolts bolted walls to roof, to cinch them together as well as PL adhesive) were coated with PL, and capped with stainless acorns. They never leaked when poly mix was liberally splashed over them, nor when painted over. Still, more sealant was applied. But, the sidewall piercings, both for full thru-wall bolts, and vent and trim partial piercings, were either sealed in the same fashion or were overlooked in the heat of the build. Either way, my initial plan was to go back, after all the poly, paint, adhesive, and thread sealant had cured, and use a clear acrylic or enamel spray to cover all skin piercings (nuts, bolts, screws, washers, et al...). I initially intended to use Krylon Acrylic Clear

krylon.jpg (23.87 KiB) Viewed 5211 times

http://www.krylon.com/products/crystal_clear_acrylic/, and I tested it when I painted my HHR's wheelcovers 18 months ago. I painted them white, to match the panel van's paint, and clearcoated with the spray. It wore very well. I recently was at Home Depot though, and talked briefly with a Rustoleum rep, who said that their Rustoleum Clear Enamel

sr_crystalclearSP_L.jpg (9.74 KiB) Viewed 5211 times

http://rustoleum.com/CBGProduct.asp?pid=166 would work as well. I'm leaning towards the enamel, where it will contact the enamel paint, and the acrylic where it only contacts metal. Sound like an OK plan?

[qwerty11]

Could you guys provide me a link to the CPES, tape, and epoxy at Lowes or Home Depot? I can't seem to find it on their website.

[les45]

Could you guys provide me a link to the CPES, tape, and epoxy at Lowes or Home Depot? I can't seem to find it on their website.

You won't find those products at the big box stores. I bought my CPES at Jamestown Distributors online and my fiberglass tape and AeroMarine 300/21 epoxy kit from AeroMarine Products online. Each of those companies had the best prices at the time about a year ago. You can Google the company names to find their web sites.

[catleppert]

bump - marking this for hubby to read later. :-)

[Donutboy]

If you want to consider going the epoxy route, West Marine carries the West Systems brand. West Marine stores are common in the Texas cities I've lived in.

[Kharn]

If you want to do as little finishing work on the wood as possible, consider getting MDO plywood. Its designed for exterior building signs and such, so it can survive in the elements for years if you treat it with a few coats of paint per the manufacturer's recommendations. Olympic Panel (which is what the local yards carry in my area) specifies two coats of primer and 2 coats of paint, both must be 100% acrylic latex (basically the most expensive exterior latex paint your big box store will carry, vs the cheaper interior house paint which is not 100% acyrlic, but always check the manufacturer's specifications before buying).

[Kody]

I have mentioned this before (a very long time ago) but I will offer a gentle reminder of how to fiberglass the corners.
I have never used F/G tape more than once to seal an external or internal corner, it was a disaster. The doubled fibers on the outer edges of the tape leave a horrid ridge that has to be sanded and faired back to the original surface. Not fun by any stretch of the imagination. Secondly, only half of the threads are holding the panels together. The threads that run lengthwise offer no strength at all to the joint or edge. A much better way is to cut a strip of cloth, 4 or 5 inches wide from a roll and cut the strip on the bias. That is, cut the strip at a 45 degree angle across the width of the roll. When this is now applied to the corner, both threads of the glass cover the overlap at 45 degrees. This offers vastly superior strength and is very easy to get the F/G to bend over the corner and stay where you want it while the resin is curing. Don't forget to apply a sealing coat of resin before placing the F/G strip on the seam/joint. The sealing coat must be fully cured so you can place the strip easily. You can use two coats to seal the wood as this is where the edges of the strip are sanded down to flair into the panel/s with a minimum of fuss. This method is by far the very best to use for interior corners. The glass will push fully into the corners without springing out. The edges of the external corner should be sanded or routed to form a 1/8" or 5/32" radius. This make laying the F/G around the corner "a piece of cake". If you want a square corner, an extra strip of glass can be applied to build up the edge and then sanded back as needed. If you want/need more than one layer of glass, cut the first strip 4" wide and the second strip 6" wide. This will flare into the panel much better with a minimum of sanding to finish it. This is how I will be building the sides, roof and inside joints of my TD.

Kody