It's understandable to be concerned about the potential for chemicals from the preservative and paint to leach into the moss and plant. The type of preservative and paint you choose can make a difference in minimizing any potential risks.
For the preservative, choose one that is labeled as safe for use around plants and is rated as non-toxic. There are several options on the market that are specifically formulated for use on wood that will be in contact with soil and plants, such as copper-based preservatives or borate wood preservatives.
For the paint, select a paint that is labeled as non-toxic and safe for use on surfaces that may come in contact with food or water, as these will be less likely to contain harmful chemicals. If you decide to apply a clear coat of polyurethane, be sure to choose one that is labeled as food safe or non-toxic.
It's also a good idea to allow the preservative and paint to fully dry and cure before adding the moss and plant to the mounting boards, to minimize any potential for leaching. With these precautions, you should be able to minimize any potential negative effects on the moss and plant.
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I'm going to mount a couple of Staghorn 'pups' to plywood mounting boards. I plan to treat the boards with a preservative to protect the plywood for the VERY long haul (time) and then coat it with two coats of paint for 'looks'. Of course, the 'pup' will sit on a base of moss and peat moss and then tied down appropriately. What I'm conserned about is whether or not the preservative and/or the paint will 'leach' into the moss and plant and cause long term ill effects. I could, if needed, coat the paint with a clear coat of poly or such. None of the sources have addressed this so I'm asking here.