How often should a deck be stained or sealed in the PNW?

Semi-transparent stain lasts 2–3 years on a PNW deck; solid stain lasts 4–6 years; clear sealer lasts 12–18 months. Whichever you use, plan on a full clean and re-coat schedule and stick to it — a deck that's gone gray from neglect needs more than a re-coat, it needs board replacements first.

Why are my deck boards cupping upward at the edges?

Cupping means one face of the board absorbed more moisture than the other — almost always the underside stayed wet longer than the top. Improve airflow underneath, keep the ends sealed, and stain both faces if you're re-doing the deck. Severe cupping (over ~3/8") is a replacement, not a fix.