I guess you could use a water based polyurethane to protect from
graffiti but I see a few potential problems: 
It seems the wood is already installed so the backside was probably not
sealed which could lead to cupping.
I suspect the wood will still oxidize through the urethane maybe a
little slower. 
I can't think of anything else tho except maybe laser beams a few feet
below the surface that could slice off the arm of any "tagger"!

Sorry

Eric Hammarberg
Brisk Waterproofing Company

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