One of the tasks I put off initially, but has been looming recently, is how to handle the backdrop, especially on the peninsula. It needs to be fairly thin, but also self-supporting. I considered building a “wall” using 1×4 boards laid flat, but while working on another project the week between Christmas and New Years – that involved some pocket hole joinery – realized that maybe the backdrop could be simpler. Drill some pocket holes along the edge of a piece of 3/4″ plywood and screw that into the benchwork.
Here is the peninsula prior to installing the backdrop. The backdrop will go flush to the foam on the left. The (eventual) foam on the right will then sandwich the backdrop, meaning I had to figure this out sooner rather than later in order to continue progress on the layout.
The plywood was cut into two 2′ x 4′ sections. Pocket holes were then drilled along one long edge. The two pieces were installed separately and the joint in the middle dealt with later. If I were to do it again, I might be tempted to join the two pieces together first, laying flat, and then raise it all at once.
For the joint in the middle, some adhesive caulk was spread on the edge of the first piece of plywood installed. The second piece was then butted up against it. A 3/4″ wide piece of wood was attached to the top edge with glue and nailed to hold the two pieces in alignment. The whole thing was then held in place with some temporary clamps until the adhesive caulk finishing setting.
The next step was to cover the plywood with a nice, smooth surface. My original plan was to use some 24″ rolled vinyl flashing. Joe Atkinson has had good luck using that to cove corners, based on an article by Bernie Halloran in the December 2013 MRH Magazine. My plan was to install the vinyl along the full length of the backdrop, and cove it into the corners at the end. Maybe even continuing the sheet along the walls on the opposite side of the peninsula.
Then I got cold feet. How was I going to install such a huge piece of flexible vinyl, keeping it perfectly straight and flat? I had some ideas regarding a rolling roll holder, and enlisting the help of family and friends. But before getting there, I started to wonder about the thermal characteristics of vinyl compared to wood.
Using that handy tool called the internet, I found some numbers for the Coefficient of Linear Thermal Expansion for wood and vinyl.
Vinyl = 33 * 10^-6 in/(in*degF)
Wood = 2.5 to 25*10^-6 in/(in*degF)
Since plywood is composed of layers with alternating grain patterns, and designed to be stable, The lower end of that range is more likely. What does that mean? Well, two things. First, the vinyl is going to want to expand (or contract) 10x more than the underlying wood. Hmmm.. Not great. Second, over the 16′ span of the backdrop, with a 25F change in temperature, I could expect the vinyl to change in size by about 0.2″. That doesn’t sound like much, but it could lead to waves and buckling. So, time for some experiments!
I took a 2′ x 4′ piece of plywood I had laying around and attached some vinyl to it. First, I used some cove base adhesive (Henry 440) and applied it using a notched trowel along the top and bottom edges of the plywood.
Then I adhered an 18″ wide piece of vinyl to it. and tried to smooth it the best I could.
The adhesive was still pretty thick, even though it was applied with the notched trowel. Getting it smooth was going to be a challenge as seen in the glancing photo above. On a smaller piece, I tried spreading a thin layer of adhesive using a straight blade. This worked much better and seemed to still provide enough grip, without the thickness of adhesive and resulting bumps in the vinyl.
So all is good there. I probably have a solution. Now, what happens in temperature extremes? The installation above was done at about 55F in a temperature-controlled room. I first put the plywood + vinyl in the shop (no temperature control, but out of the elements). It reached about 30F on a couple occasions (-25F temperature difference) and the vinyl showed no signs of peeling from the adhesive. Good. It was also nice and tight between the upper and lower rows of adhesive, as one would expect since the vinyl contracted at the lower temperature, pulling tight.
Next, I placed the plywood + vinyl at about 75F (+20F temperature difference from installation). Here is where problems started to appear.
The vinyl expanded at the higher temperature and started to buckle where it wasn’t attached to the adhesive. Not surprising. I expected this, but was secretly hoping to be pleasantly surprised. Darn physics – it always wins. However, where it was attached to the vinyl, the adhesive held it and kept the vinyl from expanding and buckling. So maybe some promise here, if I could figure out a way to apply adhesive over the entire surface.
Next, I went simple and asked, what would happen if I just stapled the vinyl to the plywood? Obviously it will be just as bad, if not worse, than the adhesive test above. But what if I installed it at the hottest expected temperature? Then the vinyl would always be in tension at the lower temperatures. So, with the room set to 75F (I would need to do the actual installation at a higher temperature, but this is just an experiment), I staples a strip of vinyl to the other side of the plywood.
There was some curling at the ends, probably due to the inherent curvature of the vinyl as it came off the roll, but that can probably be dealt with using adhesive just along the ends.
Next, the plywood + vinyl was again placed in the shop where it got down to about 30F (-45F temperature change). It was nice and tight at that colder temperature as the vinyl contracted and was held in tension. That’s good. But I did notice a disturbing effect between the staples (photo taken at 40F):
The tension was so great that it was curling the edges in a scalloped pattern. Hmmm… This has concerns about whether the staples would hold in the long term. Also, how much is the vinyl being stretched and is that permanent? To see what happens, the plywood + vinyl was again placed back at 75F.
Some waviness now shows up in the middle that wasn’t there originally. The vinyl likely stretched permanently. That doesn’t bode well for this method. Although the temperature extremes in this test were more than I’d experience in the layout room, I suspect long term cycling of expansion and contraction would have a similar effect.
So, what to do now? Even if the above tests had worked, I still have the problem of how to install a >16′ section of vinyl without creating ripples or bends in the process. Back to the drawing board…