I'm about to make a sliding door using a plywood back and a wood block front like the one below.
Is expansion a problem in this case?
I would probably use construction adhesive to glue the blocks in. The door will be outside but protected from rain.
This panel will be attached to a metal frame making double sliding doors. I'm in southeast Brazil, so sub tropical climate warrants a bit of seasonal humidity change.
The wood will swell slightly with higher humidity but if you don't have it in a frame and are mounting it like a sliding barn door then I wouldn't worry about expansion.
The problem with expansion is mostly for doors and windows in frames that causes them to jam.
Also, like the picture you posted, if the wood is cross-cut it will be less likely to expand and contract.
I would be more worried about the panel you are attaching them to warping then the blocks themselves.
Cool idea for a door.
Definitely, poly (I suggest One Time Wood Protector if Sun curing is possible) all 6-sides of everything after cutting & before applying, including the plywood. And, do put a single center screw (poly those too) in each piece. Glue is great, but it's rarely good on its own. The screws might add 10-pounds, but you can use a lot less glue with just dabs.
