I have made a little progress. It takes a good while (week or more?) for the TB2 to fully cure, and it needs to be fully cured for "poking a hole" tests. I've discovered that epoxy also takes several days to fully cure.
I'm testing how hard it is to force (strike) a medium size flat head screwdriver through the skin. It's totally qualitative, but I do try all the samples several times to get a good feel for it. All skins are over 1" 15 psi XPS. Note that the weights below for glued on skins (luan) include the glue weight, which in this case is PLP at ~.12 lb/sq ft.
Sample, Skin weight (lb/sq ft), Penetration resistance WAG (scale 1-10)
PMF 10oz 1 layer, .15, 1
Laun 2.7mm, .50, 2
PMF 10oz 3 layer, .45, 3
Luan 2.7+ 6oz FG, .62, 4
6oz FG 3 layer, .36, 6
PMF seems to be better than plywood on an equivalent weight basis, with the added advantage of not rotting. And if 1 layer of 10oz PMF is "good enough" there isn't any plywood you can buy that is near the weight. Multiple layers of PMF are quite time consuming though, at least the way I did it. I'd use a trowel to spread a layer of TB2 on, place the canvas and roll it down with a FG roller, let it dry for 8-12 hours, apply a layer of TB2 spread with a flat blade, let that dry 8-12 hrs, repeat. If you want a stronger PMF layer it's easier to use heavier canvas.
I was surprised by how much the TB2 evaporated. For the 3 layers I used ~.5 lb/sq ft. 3 layers of 10 oz cloth should weigh ~.21 lb/sq ft, so I ended up with ~.24 lb/sq ft dried TB2, meaning about half of it evaporated. Does this sound about right?
I initially didn't think the FG alone was that great, I would have given it a 3 after 1 day of cure, but it eventually got very hard. So hard the screw driver doesn't go through it at all, just chips.
Some other pros and cons:
Toxicity: PMF is very pleasant to work with; non toxic and water soluble. Plywood is ok, but the PLP isn't something you want to get on you. Epoxy and glass are worse. A respirator, gloves, and full suit are recommended, though many don't bother and manage to survive.
Water resistance: PMF and fiberglass should both be very good in this respect. Plywood is a potential disaster. See this:
viewtopic.php?f=2&t=49207 
Self healing: PMF reportedly will self heal dents if the temperature (sun) is warm enough to soften the glue, and I guess the foam?
I don't have any experience with it so I'll others will have to expound on this feature.
Build temperature: PLP will cure fine at below freezing temperatures, which means you can build in an unheated garage in the winter. If you want. TB2 and most epoxies like it to be >50F though you can get special epoxy that will cure at lower temperatures.
Strength/stiffness: I haven't tested this yet, but one thing I know is that it's easy to make foam sandwich panels strong and stiff enough for a small trailer, so the lightest above is fine. It might even be better if the panels are not so stiff in an impact situation, so they give rather than break.
Build time/ease: Kinda guessing here, but for a novice, plywood is probably easiest... but then you have to make sure water doesn't get to it. PMF would be in the running if you are only doing a single layer. Fiberglass isn't bad if you aren't picky about finish, but can be a lot of time, work, mess (sanding and sanding) if you are.
Cost:PMF: Since half the TB2 evaporates it isn't as cheap as I originally thought. Gallons are ~$19 with tax or ~$2.24/lb. According to my calculations that's ~.16 lb/layer (10oz wet) or $.36/sq ft. 10oz canvas can be had for $2/sq yd or $.22/sq ft, for a total of $.58/ sq ft. That's for a single 10oz layer.
Plywood: 28oz of PLP at $7.50 each works out to $4.29/lb. I used ~.12 lb/sq ft which comes to $.51/sq ft. The HD 2.7mm laun sheets are $11 or $.34/sq ft, for $.85/sq ft. If you splurge for marine ply, 3mm Okoume will run ~$50/sheet shipped, or $1.56/ sq ft, glued would be $2.07/sq ft. If you want ply I'd definitely recommend marine ply. And even then I don't think it's a good choice for the outer skin unless you use a tough seamless coating on top.
Fiberglass: US composites sells epoxy for ~$55/gal ($5.80/lb) shipped and according to reports it isn't terrible. I'm looking at that combined with 1708 cloth (17oz biax 3/4 mat) I've found for $5/sq yd. Weight and strength should be comparable to the 3 layers of 6oz FG listed above. Cost would be $1.70/sq ft total.
Other? GPW recently mentioned DuraSip skins as a potential alternative:
http://www.durasip.com/index.html The thread is here:
viewtopic.php?f=55&t=67805 Looks promising, especially if you can get 8ft wide sheets. It's like a fiberglass sheet that you glue on, so smooth seamless runs should be possible, won't rot, etc.
Filon like commercial fiberglass skin materials are another option. I've seen generic versions for ~$3/sq ft.
http://www.factoryrvsurplus.com/product ... ct_id=2122 Plus shipping, plus glue. I think these have a gelcoat layer which would mean no additional UV coating would be needed.
Many use aluminum skins too, but I know nothing about them.
by KCStudly » Fri Jan 27, 2017 9:29 am 
As we’re now adding some “patches” where ants chewed their way in , maybe sanding the exposed TB2 will help the paint adhere better ... It wasn’t a problem till we used the Extremely Sticky Gorilla Tape , masking tape had no effect ... 
