Despite really wild winds over the last couple of days I've managed to cut and fit the aluminium skin onto the lower side walls and side benches, definitely made easier by the fact that I could manhandle the panels and so only work on the flat.
In preparation for this I'd spent some time trawling through boat building and teardrop trailer building forums looking for advice on whether, or not, to use a contact adhesive when sticking aluminium to plywood. Differing rates of expansion and contraction between the ali and the substrate are the issue apparently. But, as ever with forums (except this one of course!), consensus was in short supply - every possible kind of adhesive, and even no adhesive at all, had its advocates and its detractors. In the end therefore I just decided to go with my own instinct and experience and to use Sikaflex 11FC, a one-part polyurethane adhesive that I've used many times before. It's very sticky stuff and it stays flexible and so I figure it should easily accommodate whatever slight relative movement there might be between the ali and the ply.
To give it plenty to hang on to I criss-crossed the surface of the ply with a burr tool before sealing it with a couple of coats of epoxy ...
... and then attacked the back of the aluminium with a 40-grit disc and de-greased it thoroughly with acetone.
The only down-side of Sikaflex 11FC is that in cold conditions it can be stiff and slow to spread but fortunately in the warm sun in which I've been working it was wonderfully soft and easy to apply and dispersed willingly between the aluminium and plywood when I applied pressure with a hard roller.
