I'm having a cedar trellis built over my patio and am having it painted
with a white, oil-based paint (to match the rest of the trim of the
house).I plan on having wisteria or other vine grow up over the
trellis, but my contractor told me that only a stain is recommended if
foliage will be on the trellis, because the white paint will turn
green, get mossy, etc. I live in Seattle so there are 9 months of very
damp weather.

My question is this -- is there a way to paint the trellis white to
match the rest of the house, but also water seal over the paint?

Thanks.

Mike