PMF tucked in grooves

[Grummy]

Those who have done the actual PMF process could maybe shed some light on the possibilities.

I'm curious if anyone has tried to eliminate the canvas overlap by pressing the edges of the canvas from two sections of canvas, into a narrow groove, then when dry, caulked the groove with a nice bead of urethane? Just looking for a way to make things more FLUSH.

For example, on a project with a 2" foam roof hung over the top edge of a 1/2" foam sidewall, lets say you cut a nice clean groove into the foam on the roof, 1" deep, 1-1/2" in from the sidewall. The side wall canvas goes up and over and gets tucked in. Then the center roof section of canvas gets tucked in. When Dry, it gets caulked (urethane).

I can't see any "leaking" issues with this concept, and I think the strength is pretty much still there with the canvas glued to each other and the caulk. The end finish would probably be Bed Liner.

I have not played with PMF, but I'm assume that canvas fibre does not "flare" itself for smooth curves like glass cloth does. So I don't know how far it would go over any radiused edges before bunching.

Yep, I know... I need to test this myself, but just kicking some ideas around before I commit to a design that uses PMF.

[TimC]

It sounds like an interesting idea. It may be a little weaker than overlapped canvas and glue but testing might find out if that is the case. It might be good to also try a spackle filler test to see if that holds up well. I think spackle would be easier to fill, sand and finish than rubbery caulk. I've done PMF on plywood on one build and PMF on rigid foam a couple times. Spackle has performed very well on PMF with TiteBond II and under layers of latex exterior paint.

I'll be watching for the results. Thanks for sharing the idea.

[Grummy]

I'm thinking caulk because with masking tape down each side of the crack, you could pretty much get the caulk to look like a factory seam. I would do the caulk pass at the end, perhaps after any primer coats over the TB2/Canvas aspect, but before bedliner.

I thought about using T-shaped aluminum sections into the grooves, but I don't think you can keep the groove clean enough of glue to allow space for the molding while it dries. Getting a T-Alum molding in there WHILE doing the PMF job... well, I only have so many hands.

[Pmullen503]

Those who have done the actual PMF process could maybe shed some light on the possibilities.

I'm curious if anyone has tried to eliminate the canvas overlap by pressing the edges of the canvas from two sections of canvas, into a narrow groove, then when dry, caulked the groove with a nice bead of urethane? Just looking for a way to make things more FLUSH.

For example, on a project with a 2" foam roof hung over the top edge of a 1/2" foam sidewall, lets say you cut a nice clean groove into the foam on the roof, 1" deep, 1-1/2" in from the sidewall. The side wall canvas goes up and over and gets tucked in. Then the center roof section of canvas gets tucked in. When Dry, it gets caulked (urethane).

I can't see any "leaking" issues with this concept, and I think the strength is pretty much still there with the canvas glued to each other and the caulk. The end finish would probably be Bed Liner.

I have not played with PMF, but I'm assume that canvas fibre does not "flare" itself for smooth curves like glass cloth does. So I don't know how far it would go over any radiused edges before bunching.

Yep, I know... I need to test this myself, but just kicking some ideas around before I commit to a design that uses PMF.

PMF works because it forms a continuous tension membrane around the foam. The groove would potentially create a weak line. I'd test a piece to destruction with and without the groove.

[QueticoBill]

This! The strength is from the skin. All the foam does is hold the skins apart, like the web of an I-beam or the webs of a tress. Any stress on that joint will pull it apart.

[Grummy]

>>> Any stress on that joint will pull it apart.

Yep, I realize the value of the continuous canvas. If the groove width is perfect, the two flaps of canvas will be bonded, and any one who has pulled apart something caulked with something like ISO-Quad would have to admit, its not exactly easy.

But, to be clear, I am not planning on an 'only foam' structure. There will be minimal framing. I plan to use foam on the exterior for the ability to round edges and still get a uniform exterior finish. My wall layups will be 1/8"interior ply on 3/4" (flat) cedar, filled with 3/4" pink foam and finally, 1" pink foam on the outside, 1-1/2" or 2" on the roof. All foam joints will be properly glued and radiused before PMF.

Some think that 3/4" studs laid flat in the walls isn't enough, but once interior cabinets and or horizontal shelving is in place, that increases the structure of the wall considerably.

PMF might not end up working for me if I cannot avoid visible overlaps.

[Pmullen503]

That's a different situation. Still need to test to see it you can get the finish you're after.

[QueticoBill]

If it relies on framing for structure and appearance is a priority, not sure I see the reason for canvas finish. It would not really be pmf.

[Grummy]

>>>>> not sure I see the reason for canvas finish.

So far, I've only seen 3 methods to seal up or skin pink/blue foam board. 1) Glue/Canvas/Paint, 2) Epoxy/Glass Cloth/Paint and 3) a High End Urethane spray coating that sets up as a hardshell.

PMF is obviously less expensive, especially when some are reporting to use latex Kilz II instead of TB2. Smooth finishes seem to be most accomplished with cotton sheets or landscape fabric. I don't think it is much to ask if your are going through a build, to end up with fairly smooth panels. Todays costs on wood and foam is pretty hefty. To put something that looks like used shag carpet over it wouldn't make me happy.

So, it's either this to get nice radiused corners or I'll have to stick with all square edges everywhere.

[Pmullen503]

QB has a point about foam and canvas if you are after a smooth finish. You'll definitely want to do a test on like a 2 ft square piece to see if you can get the finish you want.

I'm getting ready to start another trailer but it's an RC airplane hauler. All wood with 6" radius corners done with cedar strip technique and fiberglass. The entire top is hinged clamshell style. Surface finish is important and the ability to flex without cracking.

The point is my foamy camper is 11 years old with thousands of miles on it but it would be a lot of work to get the kind of finish I'm looking for with my wood and glass plane hauler. And there are ways to get radiused corners in wood.

[QueticoBill]

I also wondered if partially framed if joints or seams between frame and foam won't telegraph through the PMF? It seems PMF construction generally is all foam and effort to smooth joints between foam is required for a smooth finish. Maybe someone with PMF experience can say.

[Pmullen503]

If you have wood flush with foam under the canvas, it will eventually telegraph through due to different thermal and humidity expansion. Only took one winter on my foamy. But with 100F in the summer and -20F in the winter that's a pretty big range.

PMF over foam has it's advantages but a flawless finish is not one of them. Seams, telegraphing and the texture are part of the bargain. If that's not OK, you should work with a system that intrinsically gives the result you are looking for.