Being a school with rambunctious kids roaming around looking to tear something up, or down, I had to make sure these panels would be sturdy. Didn't want any fasteners on the front, so they are all on the backside. Panel is half inch aluminum over plywood. Bracket is 1/8" Aluminum Angle. Screws in the panel are 5/8" Stainless Steel, with a washer ( I nipped the tips off and predrilled slightly), so there is no chance of penetrating the first surface. Screws through the vinyl posts and into the 4"x4"'s are 1 1/2". Made several of these around the 2300 student campus about five years ago. People are still finding their way around to this day. It's not a mantra, but I you can't make pretty signs, then don't make signs. P.S, I didn't spec the posts.
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