Cracks forming in trailer skin...why?

[cracker39]

After building two trailers, I have a puzzling situation and wonder if others have run into this problem. On the Pop Top’s front upper panel, some cracks began appearing in the paint as well as on the top of one of my two tongue boxes. Careful examination shows no other cracks anywhere else on the trailer.. This is just about two years after completing construction. Here’s why I find it puzzling.

First, what I thought was a very careful finishing of the luan plywood skin. I sealed the corners with cloth and polyester resin. Then, I applied two coats of polyester resin to the.plywood’s surface, sanding lightly after each coat. Finally, two coats of primer and two coats of latex porch and deck enamel paint. Except for one washing and one exposure to rain, the trailer has been kept under my portable garage tarp. The ends were open and with strong winds, rain can blow in onto the front which is closer to the end of the covering, but little blows in on the back.

The cabin walls are insulated with 3/4” foam that I thought would prevent moisture from humidity from penetrating in from the inside. I always leave the top vent open and my windows open so air can circulate through. I thought that would be the proper thing to do but now wonder if leaving the cabin shut tight might be better to keep Florida’s high humidity out (?).

The cracks in two places are in the plywood as well as in the paint, and are up to 6” long. With the paint off, I can feel the edges of the cracks with a fingertip and a corner of my metal scraper/spreader blade will penetrate a small way in. There are several other small areas that look like cracks may be forming. Each of the two photos below show an area about 8"-9" long





Any ideas on why or how these cracks started forming in the wood after all of the sealing I did on the plywood?

I’ve removed nearly all of the paint on this panel with heat and a scraper and am sanding off the bits remaining. My plan is to cut a small groove about 1/8” deep and 1/8” wide along each crack and coat them with wood hardener, sanding the area around the crack lightly after it cures. After a couple of coats of wood hardener, I plan to use polyester filler on any scratches or gouges in the panel. When the panel has been sanded smooth again, I’ll apply two more coats of polyester resin, primer and paint.

Have I gotten anything wrong in my approach to refinishing the panel?

[TPMcGinty]

That happened to mine. The marine varnish wasn't enough to keep the water out of the plywood. I had to completely redo the finish with epoxy resin and camo cloth to hide the water damage.

[Larry C]

Polyester Resin is not very water proof on wood. To fix your problem, I would use EPOXY and a thin layer of glass, 2-4 oz.
Unless the plywood is delaminating, the Epoxy should seal it, and the glass will stabilize the outer ply layer from cracking.
Is it inexpensive plywood with voids? maybe moisture is coming from the inside out.

[cracker39]

What I referred to as Polyester resin is Bondo Fiberglass Resin. It's advertised as "100% Waterproof, Strong, Durable for interior and exterior home use. It comes in quart and gallon sizes with tubes of liquid hardener. I've used it on two boats and a kayak that I made. The exterior of the hulls also had glass or polyester cloth with the resin soaked through it. I didn't see the need for cloth on the trailers except for the corner seams. I'll put a layer of cloth on this panel before I I seal it with the resin.

[doug hodder]

I'm not an expert when it comes to stuff in your environment, but like on a boat, maybe encapsulate the wood with epoxy. I did the interior of my tongue box as well as the exterior with the epoxy. I've had it fill with 1" of water from rain and no issues. I've seen people do the outside of a boat only and then wonder what happened when the outside lifts. Moisture penetrated from the inside out. Maybe it's the same sort of thing. Too much humidity on a side of the wood that isn't coated out. Doug

[wagondude]

The cracks are checking of the plywood. It transmitted through the resin because the resin was unsupported (no 'glass). Polyester resin is fine for stuff that won't be submerged, but epoxy is better. Using 'glass with the resin not only supports the resin, it also reduces the chances of the plywood checking by supporting it as well. Even with good sanding, you may find adhesion problems if you try to use the polyester again. Epoxy will give a better bond over the old polyester.

[cracker39]

Epoxy will give a better bond over the old polyester.

What type of epoxy, where do you buy it, and in what quantities do you find it?

[S. Heisley]

I'm sorry that you are having problems with your trailer plywood, Dale. However, with your smarts and abilities, I'm certain you will find the solution and get it fixed good as new.

[tonyj]

Boating and woodworking supply stores carry it, and we're not talking about the little cartridges of 5 min epoxy. Two part epoxies with resin/hardener ratios varying by manufacturer are much more effective at water repellency than poly resins. Poly tends to sit on the surface while epoxy migrates into the wood fiber. Available in pints, quart, gallons. More expensive than poly resin (high end non-yellowing epoxy can run 80-120 dollars a gallon. Major manufacturers are West, System 3 (my choice), Raka, MAS to name a few. That said . . .

As someone else posted above, poly (as well as epoxy) that is just brushed on without cloth has more tendancy to allow the underlying wood to check as it goes thru dry/moisture cycles. I won't use poly resin on a wood project anymore. I always use epoxy. I have yet to have an epoxy/cloth panel failure due to moisture where I have had lots of poly/cloth failures.

[rowerwet]

I had a similar defect in a boat I built, turned out there was a defect in the tree when it was peeled to make the ply, they fill these voids with what looks like fine sawdust. I had to cut the top open on the defect, remove the sawdust, then fill the whole series of long thin voids with thickened epoxy. after sanding the defects don't show and the boat hull is stronger.

[Larry C]

What I referred to as Polyester resin is Bondo Fiberglass Resin. It's advertised as "100% Waterproof, Strong, Durable for interior and exterior home use. It comes in quart and gallon sizes with tubes of liquid hardener. I've used it on two boats and a kayak that I made. The exterior of the hulls also had glass or polyester cloth with the resin soaked through it. I didn't see the need for cloth on the trailers except for the corner seams. I'll put a layer of cloth on this panel before I I seal it with the resin.

Dale,
I am familiar with the product you used. A friend made a below the rail plywood toneau cover for his Pickup. He used Polyester resin, and 6 oz glass, and then painted it. It lasted about 2-4 months of rain and sun. I helped him repair it. We grabbed one corner of the glass with a pair of pliers, and it pulled off easily with very little damage to the plywood. There were some spots of wetness and mold under the glass. He removed the mold, waited till the next weekend, and sanded it smooth. We reglassed it using epoxy, and that was 4 years ago. It is still on his truck. It is looking pretty beat up now but it hasn't delaminated from the plywood. I know epoxy is a LOT more expensive, but with the time and effort in building a trailer, I think it's worth the expense.

[atahoekid]

If you decide to go the epoxy route (which is what I think I'd do) I'd recommend using epoxy from AeroMarine. Here's the link to their website: http://www.aeromarineproducts.com/. Their products aren't as expensive as the others that were mentioned, but they sell only on the internet. shipping for me was quick (often just a couple of days from San Diego to my home in Northern Nevada, not sure about Fla. Things shipped UPS from Florida take about 10+ days to my house) I found their products to be of high quality and I used them on every part of my Road Foamie build. No regrets!! Just my 2 cents...

[Roly Nelson]

Bummer, I have no idea what caused the problem. I was concerned about delamination when I built my woody 10 years ago, but have applied about 10 coats of spar varnish on cheap Lowes lauan plywood, (ONE PER YEAR), and so far have not experienced any moisture problems. Hopefully someone can give you some answers, but other than a bunch of coats of varnish, I don't have a clue.
Roly

[GPW]

Had the same thing happen on my wooden TD ... could have been saved or prevented in the first place by just covering the areas with a light canvas , well painted ... like on the old canoes ... Easy fix !!!

[rowerwet]

what my boat hull problems looked like after filling with thickened epoxy