Ive used sealant on decoupaged items. Best I've found is an Acrylic Matte Medium which I thin with water. It is white liquid which dries absolutely clear, no shine, and preserves true color of dried botanicals. It cleans up with water too but once dry is pretty much mosture proof. I've decoupaged wooden trays and kitchen canisters which I can just damp-sponge...I'm not sure I'd actually wash/soak them. The medium can be used as glue too.
I use Liquitex brand but any brand would probably be fine.

I've used this medium on cards to go thru mail...works fairly well but I really don't send many cards. If I did I'd experiment by sending some to myself to test.

I've done a couple of wooden plaques which were fine but mostly now I leave my arrangements unsealed..just use acrylic box frames to protect them temporarily and change often. (Or no cover of any kind...often last beyond the time I tire of them or have new material I want to experiment with...plus I often add a few seedpods or bits of bark, moss, etc. to make it more 3-D than flat).

A friend used wax to decorate candles last Xmas..she used some of my pressed Japanese Climbing Fern to coil around the candles and they are lovely. I haven't burned the candles, though...not sure about safety. josh