Need Help From All Of You Experienced PMFers

[Jwh92020]

I have gone back and forth on what to cover my square drop with. When I thought of this project, my plan was a 4x8 covered in PMF. I ended up with a 5x8 and looked at every conceivable material and I've settled back on PMF.
Questions:
Is 7 oz canvas heavy enough?
Is it absolutely necessary to wash the canvas first? (I see some do, some don't)
Sidewalls are 3/4" ply (it was free), front wall & roof are 1/5" underlay plywood on 2x2 framing, Rear hatch exterior will be 1/5" ply on 2x2 frame filled with 1.5" foam board and 1/8" ply interior skin - Should I put a 1/8" round over on all the square corners?
Straight TB2 or thin it?

I don't want this to end up looking like some of the horror stories on Youtube, so any advice is welcome.

 

[Pmullen503]

If you are building plywood trailer consider using oil based paint. It's more waterproof and UV resistant than latex. In a marine environment especially, oil based paint is prefered.

Yes cleanup is a PIA but equipment enamel is no more expensive than latex paint though your color choices are limited.

That said, enough coats of latex is still good and canvas soaks up a lot of paint. You can often find "oops" cans of mismixed color at a discount for your early coats.

Check your outer skin plywood to see how waterproof it is. I had bad experiences with underlayment. After several years it delaminated under PMF and latex paint. Toss a piece in a bucket of water for a couple days to see if it delaminates. Underlayment is smoother than exterior grade plywood but the glue is not anywhere near as water resistant.

Definitely round over your corners as much as you can, it will be easier to get the canvas to wrap around the corners without bubbles.
7oz. canvas will be plenty and be easier to fill.

Unwashed canvas shrinks more than washed canvas which isn't a problem if you plan for it. Washed canvas is softer and easier to get around tight corners.

I wouldn't thin TB more than 10%. That reduces viscosity significantly and won't reduce the bonding strength that much. You could also use primer paint if you want something less viscous.

Plan where your seams will be, they will show.

 

[Jwh92020]

Thank you for the info. I had some strips of underlay laying on the side of my house waiting for the big trash day. It's been rained on at least 10 times in the last month. I looked at it this am and it hasn't delaminated (which is why I picked some up today). As for shrinkage, the trailer walls are 48" x 96" l, rear is 48" h x 61.5" l , I got 63" wide cotton duck and figured the fabric at 6" longer than each wall. I got a piece of 72" wide to do the roof, slope and front wall in 1 piece. The canvas is about 16" longer than the length of the roof, slope & wall. I wasn't going to thin the glue unless it was considered "necessary". Wouldn't shrinkage be good as it would pull the matetial tighter to the wood?

 

[Pmullen503]

...........Wouldn't shrinkage be good as it would pull the matetial tighter to the wood?

Yes it is.

If you wrap the canvas around a corner (and make sure it's well stuck down) the material will tighten more than shrink. That can help keep the corners tight.

When I did mine I build a jig that I could run along the top that held a utility knife blade 3"down the side so I could trim the top canvas and get a nice straight seam line. If you wait until the glue is partially set up, the canvas will cut clean and not unravel. Then you just peel off the excess. It's tricky because you don't want to cut the canvas beneath but I went slow and got a good result.

Another time I had wide enough canvas to go from the bottom of a side to the center of the roof, butt joint along the center line on the top, and cover that seam with a 6" wide strip. Since the top was tall, you don't normally see that joint.

Seams are just a part of the PMF process but if they are done well it looks OK.

 

[Jwh92020]

My side pieces are wide enough that I could wrap up onto the roof a good 10" and my roof piece will cover the whole roof and has a 4" overlap on each side. I want to end up with a 1 1/2" seam at the roof/side wall and the slop&front wall/ side wall. I don't think the look of having the sides overlap the roof, then the roof overlap the sides will be an issue as there will be a 39" x 65" solar panel on the roof that will hide most of it.