Help me with walls and roof

[QueticoBill]

There may be too much great info here. I've narrowed my choices to a ply and foam core system, still weighing 1/8" vs 1/4" ply or a combo, and weighing reliance on the foam to skin bond or on "framing" bond for strength. I like the elegance of practically no framing but the less fussy aspects of the stressed skin panel using wood webs.

But my questions are really about coatings.

I may go to spar varnish, because I like it, have a lot of experience including windows and doors on my house as well as spar on my canoes. But I sometimes think about wanting color and using less expensive ply without grain showing.

So, basic is just house paint or perhaps marine top side poly. (Bedliner seems definitely pricier.) Is conventional wisdom is to use "the mix" of varnish and mineral spirits under this, no primer, and gloss alkyd top coat(s)?

If instead I went pmf route, since I can probably get fabric free in my business, is any "primer" under pmf recommended, or just TB on raw ply?

Make sense that pmf doesn't need the roof edge trim and just paint on ply does?

How does gloss work on pmf? I generally like flat or satin finish coatings - more tolerant of imperfections.

If it matters, planning home built doors and wylie windows.

[daveesl77]

Our walls and roof sections are built differently. Our flooring we way overbuilt, the walls much less so, but are still super strong - like crazy strong.

Walls are 1/4" ply (inside and painted), then the frame is 1" x 3" spruce with all voids filled with blueboard xps foam (about 98%+ of the structure), then there is a 1/8" ply and then covered with 3/16" cedar strips. All wood was fully sealed using the "mix". Exterior of the cedar has multiple coats of spar poly.

Front/Roof/Rear is slightly different. It uses a layering approach too, though. Front/Rear are PMF outer covering, using TB2 and multiple layers of exterior semi-gloss latex paint. Used duck type canvas I got from Walmart, it was 72" wide and only cost like $2 per linear foot. Roof is 60mil EPDM, white bond. Had I known how good the PMF works, I'd have done PMF on the entire structure and not used the EPDM, which would reduce cost and weight. Under the PMF/EPDM is 2mm ply, sealed all sides with the mix. Next is 2x2 spars, spaced every 8", with XPS foam filling voids. Spars are held with screws and glue and top section sits on a 3/4" wood lip. In addition there are four, 1/4" all thread rods that put compression on the walls/spars. Inside is another 2mm ply sheet, sealed. Then I glued on the thin sheets of Polywall, just to have a bright, white plastic surface that is easy to clean.

All foam is glued to all sections. The complete outside walls could be easily picked up by me and moved around, I doubt they weigh 50# each. Front/Roof/Rear section is even lighter. My build gallery shows every step I used.

dave

[dmdc411]

I used 1x3 poplar for framing the side walls, all laying flat. Filled all voids with pink high dens foam, and glued for structural aspects. Framed door, window and any attachments points. Alum skinned with 3/8" plywood on exterior, and 5mm luan inside. Way strong and heavy! I'd still go 1/4" ply on the exterior, but use 1/8" inside. Gluing the foam and skins creates a real strong torque box affect. I floated my aluminum skins. They're fine. Build light weight, glue everything. Seal the heck out of it to reduce leaks, it'll be fine.

[QueticoBill]

OK - TB2 or PLP for your foam to ply adhesive? I'm liking TB2 - for everything - but not sure it will dry through the ply.

[QueticoBill]

I have narrowed options - a little bit - but trying to see the big picture. Ply and foam walls. The 14" (5mm) vs 1/8" (2.7) is still a struggle. Right now near me it's 10.99 vs 9.99 a sheet - so that is not a factor. Weight to some degree is to be considered but at 18# vs 11# a sheet, I estimate only 1/4 pound per square feet and using around 210 sq ft - 50 pounds. Not a deal killer. I did feel more comfortable going to a sparless roof with the 1/4" - a nicety but not a necessity.

3/4" foam in walls and 1 1/2" in roof seems reasonable.

So the adhesive - PLP, TB2, NF30, epoxy, ? NF30 seems technically right, no drying issues or time, no clamping or weight needed past a quick steel roller or hammer; but not so popular here; temperature and dust sensitive; and I think would require a different glue - TB2 - for frame. PLP might have drying problems, seems messier, requires a different glue for frame, but may be least expensive; as well PLP seems not sensitive to dust but probably some issues if too cold; required clamping or weight and tine to dry. I'm uncertain TB2 will ever dry through the ply but is less expensive, easy to use, and can be used on frame as well - what seems like a distinct advantage when rolling on the coat for the second ply, probably doesn't mind dust and works at lower temperatures; requires clamping or weight and time to dry. Epoxy has some greater expense, probably more supplies and messiest; definite temperature sensitivity; but is probably most waterproof, as "strong" as any, and can be used for wood as well as the foam; requires clamping or weight and time to set; and I guess its OK with foam - doesn't melt it. All except the NF30 seem to benefit from teh roughening with a carpet roller or similar.

What did I leave out or get wrong? I'm not sure if any adhesive requires or benefits from pretreating the wood with "the mix" or CPES or other.

It doesn't seem the adhesive selection can be completely separated from the coating. I am leaning towards an opaque coating - good house paint (check out Aura) or a marine polyurethane "upper deck". I could revert to clear but would like to forego the attention to protecting the surfaces and avoiding seams and fasteners.

If I went the epoxy route, it seems that just clear epoxy, or epoxy over fibreglass over all exterior, or perhaps epoxy over fibreglass just at seams? I don't know if for instance a bias tape glass along roof/wall and rest just epoxy coat blends well with an opaque coating. Leaning towards roof and seams, but no glass on walls, and paint. This choice would lead me towards using epoxy for the wall construction - a kind of all epoxy all the time approach. I think I read that the epoxy without glass prevents the fissures or cracks in ply that let in moisture, but not sure. Also epoxy fillers work well for fasteners and poor seams with this.

I guess "the mix" and house paint is an option, least expensive, but some doubts on roof and checking or cracks. It would seem the classic trim at roof edge is necessary at least to protect that joint.

Then there is PMF. It would make this ab all TB2 all the time situation, not a bad thing. (5 gallon pail?) Eliminates clear finish option but I think works well with house paint. I can't tell if I can rely on this for roof/wall seam. I also have doubts about my ability to get it smooth, especially at the roof/wall seam.

The epoxy would also provide under side coating; whereas Henrys or one of those black gooey coatings would work better with the other approaches.

Just to complicate, the PMF or glass on walls doesn't seem to work well with my idea of home made doors - basically a flush door with a wood frame and jamb, rabbet for overlap, and silicon gasket.

Leaning towards epoxy but awaiting sticker shock. That I also have two kevlar canoes to refinish is just more reason to do epoxy. Being in an often not warm climate is against it.

I know there are some strong preferences and opinions here, so will never find a design that all endorse. Mostly want to avoid show-stopping mistakes and do it as inexpensively and simply as possible. This is mostly based on much that I have read here but trying to move forward. Must be the unseasonably warm weather here in upper midwest.

Have at.

[KCStudly]

If I had it to do over, now that I have more experience using epoxy on my build, I would have used it more exclusively from the beginning.

Everywhere I used 30NF (sometimes with questionable results and long dry times) I could have used epoxy for the same price.

I try to do all gluing before sealing or finishing, especially for wood and wood glue. Where I did not want glue splotches spoiling the wood finish, I either masked extensively or scrape/sand very well before staining.

[Atomic77]

If I had it to do over, now that I have more experience using epoxy on my build, I would have used it more exclusively from the beginning.

Everywhere I used 30NF (sometimes with questionable results and long dry times) I could have used epoxy for the same price.

I try to do all gluing before sealing or finishing, especially for wood and wood glue. Where I did not want glue splotches spoiling the wood finish, I either masked extensively or scrape/sand very well before staining.

I agree with KC... Epoxy is the way to go. I would go as far as to say the "right" way to go, but that is coming from someone who has literally used hundreds of gallons of the stuff.

[QueticoBill]

So would you use epoxy for foam to ply, ply to ply, foam to foam, as well as an exterior sub-coating with spar or enamel over?

And CPES as a primer? That seems to be the wood boat regimen.

[Atomic77]

So would you use epoxy for foam to ply, ply to ply, foam to foam, as well as an exterior sub-coating with spar or enamel over?

And CPES as a primer? That seems to be the wood boat regimen.

Pretty much. Sometimes epoxy and some foams don't play nice together so you'll need a test run to be sure. In the race boat world we've used epoxy in all these scenarios, with the right materials of course.

[KCStudly]

For cabinetry and other wood-to-wood joints that will be well protected by subsequent operations I like to use TB2 wood glue, but epoxy should work most everywhere else.

[QueticoBill]

Thanks. I was really thinking about applying the second ply skin over framing and foam. Seemed simple to roll one adhesive over frame and foam and set skin.

[Andrew Herrick]

OK - TB2 or PLP for your foam to ply adhesive? I'm liking TB2 - for everything - but not sure it will dry through the ply.

Someone else chime in with a different experience, but I've tried to glue plywood to both extruded and expanded polystyrene foams with TiteBond II, and it certainly did not work.

On the other hand, I've tried different polyurethane glues and foamboard glues, and they worked well. The polyurethane glues required a light misting of the foam; the foamboard glues required several hours of clamping pressure and lots of air drying time.

Sure made me wish I had a 4x8 vacuum bagging system

[Atomic77]

I did a 10x20 vacuum bag. 4x8 would be easy...

[QueticoBill]

OK - TB2 or PLP for your foam to ply adhesive? I'm liking TB2 - for everything - but not sure it will dry through the ply.

Someone else chime in with a different experience, but I've tried to glue plywood to both extruded and expanded polystyrene foams with TiteBond II, and it certainly did not work.

On the other hand, I've tried different polyurethane glues and foamboard glues, and they worked well. The polyurethane glues required a light misting of the foam; the foamboard glues required several hours of clamping pressure and lots of air drying time.

Sure made me wish I had a 4x8 vacuum bagging system

Yup. So now I'm looking at moisture cured polyurethane adhesives that come in gallon and larger containers instead of cutting open tubes. Thanks.

[JaggedEdges]

OK - TB2 or PLP for your foam to ply adhesive? I'm liking TB2 - for everything - but not sure it will dry through the ply.

Someone else chime in with a different experience, but I've tried to glue plywood to both extruded and expanded polystyrene foams with TiteBond II, and it certainly did not work.

On the other hand, I've tried different polyurethane glues and foamboard glues, and they worked well. The polyurethane glues required a light misting of the foam; the foamboard glues required several hours of clamping pressure and lots of air drying time.

Sure made me wish I had a 4x8 vacuum bagging system

Odd, it's the same type of glue model aircraft guys have used for years for foam to wood. Best stuff to repair a broken foam wing too.

However, it's moisture activated and there's a possibility in a huge panel that there's not enough moisture available to kick it off.... would suggest maybe sponging the plywood down so it's very lightly damp.

You can just use tuck tape and heavy polythene sheet to bag anything, use dollar store vacuum clothes storage bags for the valve thing to ghetto rig it with a shop vac, won't be perfect. Another approach might be to get a large enough kiddie pool, sit it on top and fill with water. However, either of those methods leave you dealing with hardened seeped out glue if you're going to leave it in for full cure and also remove all that spare humid air you might need to cut set time.

Edit: derp, tightbond polyurethane that's moisture activated, reading badly worded stuff that talked about II and III in the same sentence and thought it applied to all.