I spent the last week or so
making a huge mess from little pieces of foam bending and gluing building foam into the roof sections, like this one:
Of course, there are wires everywhere, so I needed to cut out the appropriate channels like on the right. This particular piece was a curvy one, so it needed to be ripped partway through so it would bend, like this:
It all went far better than I thought, honestly. I expected a lot more breakage and thickness differences than I actually experienced. Here's a piece all nicely clamped, the LePage PL Premium slowly drying underneath. I feel like I've finally gotten a handle on how to use this stuff properly:
On the subject of the PL Premium construction adhesive ... I'm using a LOT of it, far more than I naively expected at first blush. Likely well over 2 dozen 875ml tubes for the whole project. It makes sense when you mull it over, since the whole structure (walls and ceiling) is foam laminated to wood on both sides. But that's 50 pounds / $300 of adhesive! Holy moly. Something to keep in one's mind when doing a laminated foam project in the future.
But it's great stuff. I'm happy with it. It does a better job than nails or screws and it bonds, permanently, to what seems like anything. It has a thickness, unlike wood glue, and it'll happily fill gaps.
"Why not use contact cement?" I hear future builders crying from their computers. Well, I'll tell you why:
- Lots of types of contact cement releases in the heat, like "summer day inside a camper" type of heat. Not awesome. The other stuff is harder to find, weaker, and expensive.
- Contact cement also tends to melt foam. So that limits us again to an even smaller selection.
- Contact cement can't deal with thickness. This was kind of a big deal for this job. The foam really is hard to get perfectly flush with the wood. It would have taken ages to sand it out before the second lamination.
- Especially when laminating thin sheets, it's hard to align them properly at first. You need some re-position time. You get about 2 hours with PL, and about 1/5 of a second with contact cement. I can't think of a good way to perfectly align a huge, floppy, 1/8" sheet of plywood before dropping it into place.
- You'd need a substantial amount of contact cement too, and it's not free either.
"Why not use epoxy?" Because it's super-expensive. The adhesive would cost more than everything else put together. That stuff is $175 / 3.6L + fillers, which would do just over one side of a 5 x 5 sheet (see below). Nope, nope, nope.
Here's the math, done unfortunately in retrospect:
For a 5 foot x 5 foot sheet, laminating 1 side with 1mm thick adhesive. Metric, with apologies to USA, Liberia, and Myanmar. 5 feet = 1.524m.
1.524 x 1.524 x 0.001 = 0.002323 cubic m
Huh. That doesn't seem like much.
But wait! 1 cubic m is a whopping 100x100x100 cm, or 1,000,000 cubic cm. Let's move that decimal place over a bit:
0.002323 cubic m = 2323 cubic cm = 2323 ml
At 875ml / tube, that's about 2.7 tubes for a 5 x 5 sheet laminated at 1mm thick. When I combed the adhesive for the outer skin (below), I used just under 3. I think that's a little more than I used when I didn't comb it, but some small voids in the walls were acceptable.
Of course, you need to do 2 sides of a roughly 15 x 5 roof and both 10 x 4 walls. That's about 37 tubes. But the area is smaller due to curves, cutouts, etc, there's some waste, and I didn't really comb the adhesive for some of the work. Right now, I dead recon on about 28 tubes, but it could be more.
Enough of that. The takeaway is: "Plan to use a lot of adhesive to glue really big things."
Here's the top:
And here's the foam all nicely placed. It all got a bit of a sanding to make it flush before I skinned it, but it wound up fitting quite well:
This is what happens to the foam scraps. It's really hard to explain to a 3-year-old why it's more important to build my little toy fort than his. So I just took his down in the middle of the night and hoped he would forget:
Finally, I got part of the external skin on! I decided to do it 2 parts, which I think was a good idea because it became much less unwieldy at the start and also much easier to "steer". The walls wound up being pretty square, but there is still a small amount (<1/8") of overlap on one side and a gap on the other. It would have been more severe if I kept going with the same sheet.
I don't have photos. I was rushed as it was.
To start, I set up a jig on which to rest the plywood, then combed out the adhesive with a notched trowel to a uniform thickness. Except for the top end (which does not land on a spar), I used 1-1/4" nails for clamping. Other than straps, which would not clamp the straight parts as tightly, I couldn't think of another practical way of doing it.
I saw the nails as little adjusters. I didn't drive them in hard at first. I tried to adjust them all until the whole piece was following the contours smoothly. There may be some valleys around the interior nails, but that can be sorted out with Bondo later. There wound up being quite a large number of nails around the edges, which was necessary to control the bowing. With plywood this thin, it was more like upholstery than carpentry!
I'm quite pleased with how it looks:
There will be some work to do to make everything smooth. Once the adhesive is dry, I'll countersink those nails and even everything out with Bondo. There's a second act as well, from the clamps in the photo to the front of the trailer. There's a bug in my brain saying that this distance might be longer than 5 feet, so there might be a little fiddle at the front to correct that problem.
I'm now debating adding fibreglass to the 1/8" skins before painting. The wall I've got seems extremely strong, but the bend will check quite easily (even under epoxy, I'm guessing) and we need to consider flying rocks from the highway.
All things considered, this is progress! At this rate, I'll be hatch-building in no time.