...and am not sure why. Any advice would be greatly appreciated.
Porosity of the material and the somewhat weak cross-link molecular structure of the substrate, lead to the failure of the vinyl application to bare wood. The microns thin layer of adhesive found on typical vinyl manufactured for use in the sign industry is not thick enough to encapsulate and seal the material. That makes it subject to premature failure from expansion and contraction, as well as environmental influences of moisture.
If there are no binding agents applied to the wood, the best explanation of the phenomenon I’ve heard...
is that your trying to apply vinyl to air.
It should be noted that air, (and even worse, wind,) is not dimensionally stable. It has a poor cross-link molecular structure and is a horrible substrate for professional vinyl application - Gravitational influences are the predominate cause of failure when applying vinyl directly to air --- Don’t ask me how I know.
It's kind of a sign making rule for me now: the more air there is between the adhesive side of vinyl and the substrate, the less likely it will adhere to the surface.
Suffice to say, key elements in successful vinyl application is determining the porosity and dimensional stability of any given substrate.
Unsealed wood is bad.
