Next phase....attaching the foam panels. I'm mindful of the fact I may have to break this puppy down and move it, someday. First step is to discretely mark where I have screws through the OSB panels to attach them to the 2x4 frame. I don't want to destroy the layout's edges, hacking away at the foam as I search for screws. A simple line will tell me where to look:
Next...theory. Humidity can play havoc with a layout: wood is a living thing, even when it's been shredded to make OSB. It swells and shrinks. One theory is to use as many screws as possible to lock it down to the frame. My own preference is to give it the freedom to do it's thing as uniformly as possible, so I use just enough screws to prevent the OSB sheets from sliding and let it float freely, beyond that. My belief is that if the entire sheet can swell move as one unit, you distribute the distortion over all of it and minumize the effect on the layout. Tack it down and the movement and distortion concentrate in the one area of least resistance. I'll find out, over the coming year, if I'm right. OSB, as TJ explained, is Oriented Strand Board. Around my neighborhood, it's often called "splinter board" because it's composed of large wood splinters formed into sheets.
Back to the table....laying down the foam sheets. I started at one end and got all three sheets to where I was satisfied: oriented properly to the table and each other. I turned them all Pink Panther side down---I doubt it makes a difference, but why give it a chance?
Materials for this step:
Foam doesn't absorb humidity and swell, but the OSB beneath it does, so the foam sheets are along for the ride. To allow that, I minimized the attachment to let them float. Two nickel-sized globs between foam and OSB on the narrow end, near the corners. For the center sheet, a glob in the middle of each short side to keep things from sliding and the sheets coming apart. On the last sheet of foam, two more globs on the short face. This will allow the sheets of OSB to move under the sheets of foam as it swells and contracts.
Credits: before I go further, I want to credit suggestions made to me by others as I planned this. Tankist was the one who inspired me to search for culls when I looked for foam---thanks, Anton! Macdaddy suggested the material for the next step (latex caulk), caulking the seams. Make sure it's latex and not oil based, and paintable.
The process is simple and fast: squeeze a long glob onto your fingertip, smear it down the seam, and go for more. Use it generously: thick works better at hiding the seam. When you get a thick smear down the seam, go back and smooth it. Mac suggested a wet fingertip; I found a tool I liked better, a cheap plastic window scraper with the blade-protector left on.
It goes on quickly and smooths well. Latex should have some flexibility and compressability to ride out the changes in the wood discussed earlier. Drying time: 2 hours.
By the way, dozer was the author of the suggestion to use Liquid Nails to attach the foam to the OSB: thanks, dozer! Thanks also to everyone who listened to my dummer ideas and patiently talked me out of them.
Drying time on Liquid Nails is 24 hours, so I'm done for today. Time to go cut grass.