Gray, splintery deck boards are usually dirty and weathered rather than rotten — but check first. Poke suspect boards with a screwdriver: if it sinks into soft wood, that board is done and sealing over it is wasted money.
For sound boards, the order that works: clean first (a deck cleaner and stiff brush lifts most of the gray), let it dry properly — two or three dry days — then sand the rough and splintery stretches. Sealer or stain goes on last, in dry weather, never onto damp wood: it traps moisture and peels.
While you’re at ground level, look at the fixings. Popped screws and nails can be reset or replaced with deck screws, and a railing that moves when you lean is worth fixing before barbecue season, not after.
Decks are seasonal work worth batching. CMG Bridge can clean up the carpentry side — boards, rails, steps across Staten Island, Monday to Saturday.