I was just going to post my piece that I've been assembling (mostly from previous writings) in between other tasks this morning and I see a couple of others have posted similar material. Seems we have much in common in our thoughts on this subject. I'd be interested to know about the wood sealer mentioned - brand and sourceâ€¦
Also, I'll comment that 'marine grade ply' (made from tropical hardwoods - that will not 'check') is now approaching $100 per sheet in the thin stuff and easily over that in many thicker dimensions, depending on variety. And while that marine ply (hardwood, not fir) will do OK with epoxy-only coating (then properly painted to prevent UV degradation of the epoxy) there is no use coating cheap pine ply, or even marine grade fir ply, with epoxy, without glass, as it will 'check' (open small cracks) and then you may as well have used cheap paint.
Glassing it quadruples the labor on the exterior, at least (and you must logically then finish the interior also) and by that time you have invested into a boatbuilding-quality system and it makes sense to use only expensive boat-grade primer and paint. The conclusion is that doing more than the standard prime/fill/caulk/paint on construction-grade materials, logically becomes an all-or-nothing proposition (one exception being the traditional 'paint & canvas deck' finish, described below).
And so I'll add my piece:
I've built, over the decades:
- An open trailer, 16' x 6', flatbed with 2' removable front & side boards with removable gate on the rear.
- Two larger (16') closed cargo box trailers with double doors on front and rear.
- A smaller cargo box trailer (8.5').
- And an 11'L x 6'W x 3.5'H windsurfing gear & 'sleeper' trailer with racks on top, welded from square steel tubing, that carry four boards, booms & masts on one side, and two mountain bikes on the other side. The interior has a permanent sail and gear storage on one side with full-length shelves holding maybe ten sails, multiple gear bags and boxes, and so on, accessible on the side through an 7' x 2.25' door that opens down and hangs on chains like a tail-gate for use as bench, gear table or spare bunk (with snap-on screened awning cover). There is also a completely separate enclosed section on the other side, with double doors on the front, side and back, that can be used as a tight double 'sleeper' section or, with the bed rolled up out of the way, steel racks that fold out from the wall and allow secure interior storage of the boards, booms and masts from the roof racks.
- You mention 'Triangular in the front'. If I understand the meaning correctly, that may not yield much, if any advantage for the work it would take, and having an opening rectangular door at each end may add a lot of advantage.
- Many people seem to tend to overbuild. For a garden shed it doesnâ€™t matter if it weighs 4 times what it needs to, but for a mobile unit that is a problem. I've seen home-made windsurfing trailer boxes made from 2x4s and 3/4" ply that must have weighed a ton, almost literally, and certainly they weighed many times more than the gear they carried.
- Unless you get into welded metal framing, or boat building techniques like glass or epoxy/wood/glass composite, you will likely be making a painted box with a wood skin on a wood frame.
- The key to light construction is using less (thinner) materials and a 'smart' design.
- Light, strong and cheap construction is easy - its preventing rot without going to expensive and labor-intensive methods and materials that presents the problem.
- To demonstrate how I learned the hard way - my first trailer box (16'L x 6'W x 7'H) was built in 1985, and it was a good design, structurally and weight-wise, but I failed to use rot-proof materials: 2x4 floor and rim joists, 5/8" ply flooring, 2x2 (1.5" x 1.5") studs, 2x2 top and bottom wall plates, roof beams sawn from 1x8 pine (gave rounded box roof top), 1/4" pine plywood for siding and roof, with hot tar/membrane roofing. It was fastened with regular deck screws and none of the wood was treated. I got a lot of daily use out of it and earned a big stack of cash hauling my tools and materials around in it while it lasted, but in a few years it was structurally rotted to the point that I was afraid to take it on the road anymore, and many of the screws were structurally rusted as well. It was then taken off the trailer, put on blocks and used, after patching it up, as a storage shed for another ten years.
My lesson from that was to never use framing, flooring or structural fasteners that can rot or rust. Treated wood framing and treated ply floor, fastened with SS deck screws was the rule for all subsequent designs. Light-weight 1/4" pine plywood used for siding and roof decking is easily and sensibly the exception to the rot-free rule, and it is cheaply patched and eventually easily replaced as needed over the years - there is no real obvious rot-proof alternative available that is all at once as strong, light and cheap. This thinking went into designing and building the next trailer box, which after eventually getting an epoxy/glass exterior with some fairly spendy paint, lives today in good and mostly maintainance-free health and is used semi-regularly. The original painted ply siding and roof lasted about seven years before I finally replaced it and finished it like a boat.
- So, for a small box, use minimal treated wood floor framing, maybe 2x2 (1.5 x 1.5), with a 1/2" treated ply floor screwed to longitudinal floor framing at close intervals for stiffness, and maybe 1.25 x 1.25 treated wall framing and top plates, with slightly curved roof beams sawn from 1x4 or 1x6 treated wood, all fastened with SS deck screws, then skinned with 1/4 ply.
I glue the side and roof ply onto the framing with foaming urethane glue ("Gorilla glue" or similar), position the ply and hold it in place with deck screws (remove later) then fasten every few inches with air-driven narrow-crown staples. Allow the ply roof to overhang the sides, front and back by maybe an inch for best siding life (round the corners and edges prior to finishing).
Sand smooth, then paint (only the exterior) with a coat of Kilz Premium primer (water-based), working it into all the holes and cracks. After the primer is well-dried, fill staple and screw holes with 'water putty' or similar and caulk all long cracks (exterior only) with at least one layer of Alex Plus 35-year siliconized acrylic caulk.
Apply two more coats of Kilz - no other paint is needed as Kilz is very weatherproof even though itâ€™s a primer - I once painted an old fiberglass fishing boat with Kilz and was amazed at how many years it lasted out in the full sun.) As with any common house-type paint over regular wood not protected by large overhangs, recoat once every year or two to prevent UV degradation and water ingress into micro cracks in the wood.
If you want a better roof (you do!) but donâ€™t want to get into hot tar and membrane, or epoxy and fiberglass, an old 'painted canvas deck' style of roofing can be used with good success on plywood: After surface is smooth and initial primer is dry, roll down a medium-heavy coat of Kilz primer (water based). Its fast drying, so plan ahead and work fast (roll most of it, just use the brush for detailing). Then lay a pre-cut and pre-fitted piece of an old bed sheet on the wet paint, smooth it wrinkle-free with a squeegee if needed (drop squeegee, and any other paint gear that is in danger of drying for that matter, immediately in a bucket of water for later easy cleanup and reuse, and keep a wet rag handy in the bucket to immediately clean your hands and any drips as they occur - much easier than letting them dry even a little). Then, before any of this dries, roll and brush on another medium coat of Kilz Premium primer over and into the cloth so there are no dry or thin areas. After it dries, put on a couple more regular coats for good measure.
Many years of keeping windsurfing gear in a wood trailer box has taught the following:
- Put in plenty of vents (from Home Depot). Front and side vents should be closable for rain travel. Cheap aluminum louvered vents with screens can be installed on 'raised' wood flanges (1" x 1" stock) with a slight 'groove' around the edge, and then to close the vent a piece of plastic tarp or other fabric can be fitted over and secured with a round turn and square knot in 1/8" line (the cheap way), or a nice cover with a drawstring and cord stopper can be sewn from light, waterproof fabric. One could even make hinged hatch-type covers from wood, sealed with weather striping and secured with latches - essentially boat hatches - but I was never that ambitiousâ€¦
- Donâ€™t paint or caulk the interior wood - let the wood of the box continuously dry from within instead of trapping moisture within the wood and causing rot.
- Keep your sails and gear off the floor and walls of the box with some sort of wood or plastic grating or other spacing material to allow air circulation all around and prevent slime, mold and mildew from growing on your box wood and your gear - this is true even in a 'fancy' non-wood boxâ€¦ I've had good success making 'cheap and easy' (like a girlfriend I once had) 'grid lattice grating' from 1/2" square treated wood stock spaced 2", then other pieces placed on top of that at 90 degrees, again spaced 2", with a staple at each junction. This '2x2' grating is an inch thick and will let air flow in all directions between gear and interior box surfaces, protecting your gear, and also the bare wood surfaces of the box interior, if that applies. Orient the top layer of the lattice long-ways for easy sliding of gear in and out of a long box that accesses from the ends.
- To keep mildew and slime under control, kill the foundational 'early stages' by spraying box interior and exterior every few months with a pump-up garden sprayer with commercial deckwash (bleach based) diluted 50/50 with water (Armorall brand from WalMart, Behr #63 from Home Depot, or just 50/50% bleach/water solution with maybe a 1/4 ounce squirt of dish soap) Let it soak in for 15 minutes, rinse well with a garden hose, then dry with a fan blowing air through the empty box for as many hours as it takes.
- When not in use, keep the painted surfaces (and your whole boat, for that matter), under a shelter or tarp (not a tarp directly on the paint). This will limit weathering to the times you are actually using it and so will extend the life and minimize maintenance many times over.