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It is not necessary to coat both sides with paint, though it will do no harm.
Local conditions can make a significant difference, if you built your panels in Arizona, and went to the wettest parts of the pacific northwest, you are likely to have surprises because your wood will deferentially absorb moisture, if only one side is coated.
I certainly wouldn't trust the foam core in the aluminum panels. Nothing made of foam has really done that well over time, so far.
>It is necessary, because otherwise it warps.Plenty of people are not having that result.>No. Humidity cycles daily, as well as seasonally. This makes it a non-rigid support. There is a time factor as well. And the degree of variance. You can't just categorically say that all daily cycles, or seasonal cycles are a problem. I buy plywood and composite products by the lift. Your comment is a vast exaggeration relative to normal conditions. I live right on the shore of Lake Ontario, and we get get a fair range of conditions. I do not have problems. You should do a wet test and see how your finish actually excludes moisture. Even 100% solids materials like boat epoxy will normalize to ambient humidity, sprays or other high solvent based materials will bead water, but are a poor buffer to humidity. >No. It's not foam, it's a polyethylene core.Fair cop, but it is still a crapshoot as to what long term stability of these materials will be. It is certainly easy to prepare wood in the home shop to an industry standard level of finish, the long term durability of which are well know. Materials like Dibond are surely superior for outdoor, and short term uses, but they are not designed for, and do not seem to seriously support fine art uses.Dibond for example clears it all up by stating that : "For oil on Dibond aluminum composite panel, each artist has their own process for prep, paint and coating the products." They sure aren't getting in the way of whatever amateur science project the artists are up to. You can satisfy yourself, but I am not aware of any industry standard process for bonding to aluminum that involves gesso, or the chemistry of gesso. I don't know what their coilcoated paint is either.I am not suggesting these oil painted panels will fail down the road. I am just not buying into the idea that there is any serious coating science behind what people are doing on them. Whereas we have countless centuries of experience with wood, and and quite a long exposure to plywood and composite wood panel products, so I am not worrying about those in favour of the latest wonder guess.