Bending Prefinished Ceiling Panels?

[KCStudly]

I want to prefinish the inside surface of my ceiling panel in the flat on the bench prior to installing it over the front radius and crowned roof of my profile.

The plywood is 5 mm marine grade Okoume with the outer ply grains running across the easy way. I'm using Minwax oil stain and 2 or 3 coats of high build interior grade polyurethane. The panel will consist of two sections butt joined with the seam running crosswise about midway in the high crown area of the roof (a gentle 520 inch radius). The seam will be backed up by a narrow slat of the same 5 mm ply.

My concerns are that if I train the ply to the curve before finishing, then sanding and applying the finish would be more cumbersome. Whereas, putting the poly on might in some small way complicate the bending; although the finish will only be on the compression side.

My front cabinets have "riblets" to help form the tight 13-1/2 inch radius front curve and give me something more to screw into. (My hybrid foamie construction method has a minimum number of spars.) I will have to do a dry fit before finishing in order to trace these areas so that I will know where to mask for glue. This will also allow me to pre-drill the screw holes from the inside after tracing so I will know for sure that the screws will find the centers of the ribs when run in from the outside.


I suppose I could layout the screw locations and masked areas accurately enough without test fitting, but there is always a certain risk in that.

So the conflicts I am struggling with are the sequence of preforming the ply by training it in a dry fit vs. prefinishing it in the flat.

Thoughts and experiences?

[tony.latham]

KC:

I didn't get your dilemma until I caught the 5mm thickness.

I think I'd do an almost-dry fit first to get an idea on how cooperative the plywood is going to be (I'm sure you have three or four cargo straps waiting for the task). But I'm guessing you've done that.

I pre-finished mine, but I was using 1/8" BB. Since my ceiling is sandwiched, and lays on the walls, I masked off the contact area that was going to be glued. But I allowed the poly finish to "protrude" into the glue zone by about an eighth of an inch. Have you considered this? You could dry fit, mask around where your mini-ribs butt, remove, and then mask the area off a bit on the skinny side to allow the poly to get beyond the ribs a touch.




You missed a good Dutch oven dinner in camp last night.

Tony

[KCStudly]

Thanks for the reply, Tony. Yeah, I have been underlapping all of my other prefinished glue joints, so yes, I have thought of that. I did do a test bend using a smaller scrap of the ply and it looks like it will take the radius fine, at least w/o any finish on it.


I do expect that it will take a lot more force to do the full 63 inch width. (The cabin is 64 inches, but I held the plywood back 1/2 inch each side so that I can seal the resulting slot at the edge of the ply, between the top of the wall and the roof foam above it, with GS spray foam, thus encapsulating the edges of the ply.)

This brings up another question. I would think that pulling on the rear of the skin would help to stretch and wrap the skin tightly to the profile. Whereas just wrapping straps from the front to the back and cinching them down could create buckles. So maybe fix the hinge spar to the back of the panel and use that as a cleat to pull the back of the skin rearward while also binding the panel down with long straps? Down fall could be that the hinge spar might not end up exactly where I need it to be to suit the prefabricated hatch ribs; so I could just clamp a cleat to the rear temporarily and pull on that instead.

Since the hinge spar will have a rabbet to cap the rear edge of the ceiling ply, I can either trim the back of the panel with a router (if it is long) or adjust the rabbet in the spar if it is short. In fact, I will probably just trim the panel even with the back of the bulkhead and the spar will overhang about 3/4 inch.

Truth be told, I am also considering gluing the three roof spars onto the top of the skin before putting it on the cabin. My thinking is that by gluing them on at the bench it will be much easier to clamp them with a strong back (I don't want any fasteners visible from the inside), and it will help keep the panel from bowing in under the stress of the straps. Remember, I'm building from the inside out, so the schedule is ceiling skin, spars, foam, then glue/canvas/paint.

After the ceiling and spars are on, I will fit arched mini ribs between the vent spars to box in the vent hole, then add the foam.

[48Rob]

KC,

I used 1/8" not 3/16 but it took the bends well and did not crack or affect the varnished finish, or if it did, not in a way that required me to fix anything.

Bends in compression, for me, seem to do well with finishes.

Rob

[KCStudly]

Thank you Rob. That's exactly the first hand knowledge that I was hoping for!

I figured it would be okay, but thought it would be best to confirm.

[mezmo]

To anchor the sheet at the top, why not butt it up to the roof
skin edge and then screw a batten through it into the first spar at the
top. That'd keep it in place as you bend it over the radius. If you
use SS screws, they could be left in place, or just remove any 'regular'
screws you use once the glue is dry.

Perhaps you could also screw on any of the other 'hidden' spars that will
go above the ply sheet and screw through them into and through the
bent plywood sheet into any other under framing you have to help position
and keep the the bent ply in place as you bend it.

Cheers,
Norm/mezmo

[KCStudly]

Thanks Norm.

I plan to have a backer slat overlapping the lower front edge of the front ceiling panel. This will be pre-glued to the panel and will overlap the front wall panel where the seam will be hidden by the front cabinet shelf ledger. To start, I will align the ceiling to the front wall at this joint, and glue and screw thru this backer and both skins on either side of the seam (top and bottom) into the ledger at the base of the upper front cabinet. From there I plan to pull on the rear of the ceiling and/or strap it down while gradually screwing up along the side walls and the front riblets until I can screw along the top of the front cabinet face frame (which acts as a spar). That gets me into the gentle arc of the roof which should not be an issue at all conforming to the profile. Next, the 2x2 cedar spars, 3 of them, that will be pre-glued on top of the ceiling skin, will get screwed down into blocking that has been strategically glued into the top edges of the walls. These spars span the main open part of the ceiling panel that is otherwise unsupported by cabinet structure. Two of these spars bracket the roof vent fan, and the other is roughly centered on the remaining space; about 16 inches c/c at the max. There are also a few blocks along the tops of the walls in between these spars for additional screws at the edges of the panels. On the final glue up I may also do a temporary backer bar and clamp set-up that would allow me to use wedge shims to get a good clamp along the top edges of the walls where there is no blocking to screw into (similar to how I installed the front wall). At the rear of the cabin I plan to screw down into the upper cabinet face frame and the top of the bulkhead wall. The cabinet face frame and bulkhead tops have been shaved with a plane to match the side profile, so I should get a nice tight glue-up.

For those of you who are not following my build thread, I had a minor set back when I found that I made a boo-boo on my cut list for the ceiling panels leaving the front panel too narrow (short), but I have a plan to work myself out of that little problem.

[capnTelescope]

I have an answer.

Those of you who have been following my build might remember this picture of me "training" a sheet of ply to skin my hatch with:


Some time after I took this picture, I did a pre-coat of wipe-on poly on the inside side of that sheet. Time passed, the sheet got set aside and forgotten until yesterday, when I got started on skinning my hatch. The ply definitely bends better the "other" way now. IIRC, I let the sheet be flat while the poly cured.

I'm not sure what the lesson here is. Either the trick is to poly the other side from the bend. or let the poly cure while the ply is bent. Or both.

Hope this helps.