PMF outer skin, layers ?

[rustytoolss]

There is so much information in "the big thread" and subject matter jumping around, I get lost sometimes . So I'm going to ask this . When doing PMF on the exterior of a Foamie. How many layers of 10oz canvas, are most people applying to the outside / exterior ?

[Pmullen503]

A single layer is adequate with overlap at the joints. Start at the bottom and work up so the higher layers overlap the lower. On the inside I found it easier to run the canvas right into the corners, cut it off, and then apply a separate 4" wide strip to cover the joint.

[rustytoolss]

At some point (somewhere) I thought that I saw a post about using 2 layers of canvas. But maybe it was someone doing a test, not sure.

[GPW]

You can use multiple layers if your material is Thin...

[dancam]

someone used 2 layers of 6oz canvass. I'm looking for who that was now

[KCStudly]

If you can get it to lay down well around the rounded corners (i.e. use a big enough edge radius for the thicker material) you will be far better off using one sheet of thicker material, rather than multiple sheets of thinner material.

One way of looking at it is that two plies equals twice as much work, and two times the opportunity to screw up. There is also a concern about applying the second coat before the first has fully cured (seems like bubbles form when the outside is sealed too soon before the inside dries). This is how I felt when doing two layers of 3/4 thk foam on my roof instead of one 1-1/2 thk kerfed layer; same idea... twice as much work and more adhesive used, had to take precautions to get it to cure.

Two layers of thinner material will likely cost more than a single layer of thicker material, especially once the added glue cost is factored in.

The other way of looking at it is material sizing and availability. If you can't find or justify the cost of material wide enough to cover a surface in a single pass, then using multiple layers of thinner material could give you opportunities to overlap seams and minimize the work needed to fair those joints down (if that is important to you). Could also provide an opportunity to alternate weaves, laying one ply on a bias and picking up more stiffness, such as on a hatch. This is the approach I took using two layers of 6oz cloth on my epoxy/glass exterior (same concepts can be applied to PMF). I could only find 60 inch wide cloth (actually 61 inch with selvage) for my 64 wide roof. By running two plies and carefully planning my overlaps I only have to deal with fairing one ply at each side of the seam, but still have triple thickness in the heart of the overlap zone.

[Andrew Herrick]

I haven't built a foamie, but I'm curious ... is there a reason to use 10oz cotton duck canvas over a double-filled canvas like 15oz? That's intended for outdoor projects, and it seems like it would require less glue and less paint to adhere and fill in the weave. But like I said ... never built one! Any thoughts?

[GPW]

All just a matter of what you can get or afford ... The thinner fabric is much easier to apply ... The thick canvas ( we used 18 oz. ) goes on more like a solid sheet , but makes a really heavy duty skin ...

PS. What we’ve found over the years now is any/every fabric needs to be “sized" ( totally saturated with very thinned glue ) to encapsulate the fibers .... any unsized canvas exposed to air over time seems to rot quickly ...