Splintered deck boards are a safety problem as much as a cosmetic one. Sand proud fibers, check for rot underneath, then protect the wood so splinters do not return quickly.
Make it safe first
Knock down sharp fibers with a block and medium grit, then finer grit. Vacuum dust. Replace boards that are cracked through or soft underfoot.
Why decks splinter
UV, wet-dry cycling, and bare unfinished wood raise grain. End grain and cut ends drink water fastest.
Seal after sanding
Apply a penetrating sealer or stain suited to decks once the wood is dry. Coat cut ends thoroughly. Softwoods may need more frequent renewals than hardwoods.
Prevention
Keep debris off the surface, fix drainage that pools under the deck, and plan a light maintenance coat before the finish fully fails.
For more repairs like this, start in the Rot & Outdoor Wood hub. If you are working through a related problem next, see Wood Decking Repair: How to Fix Cracked and Rotted Deck Boards or How to Fix Rotten Wood on the House: Exterior Siding Repair.
