I've dealt with this exact problem, and the good news is it's very preventable. Outdoor wooden benches usually start splintering when the surface fibers dry out, swell with moisture, or get damaged by sun and regular use. The key is a simple routine: clean, sand, seal, and maintain.
First, give the bench a good cleaning. Mix warm water with a few drops of mild dish soap, scrub along the wood grain using a soft brush, then rinse thoroughly and let it dry completely. This removes dirt and grime that can grind into the wood and raise the grain, which is often the first step toward splinters.
Once it's dry, sand the entire surface. Start with medium-grit sandpaper around 120 grit, then move to a finer 220 grit to smooth things out. Always sand in the direction of the grain, never against it. Pay extra attention to the seat, armrests, and any edges where hands and legs make contact. After sanding, wipe away all the dust with a tack cloth or a damp rag and let it dry again.
Now seal the wood. For a natural look, I like penetrating oils such as teak oil, tung oil, or a quality outdoor wood sealer. Apply thin coats with a cloth or brush and let each coat soak in before adding the next. If you want stronger protection, use an exterior spar varnish, polyurethane, or a deck stain with UV blockers. A film-forming finish creates a smoother shell, but it may crack over time and need more preparation to redo. Oil-based sealers are easier to refresh because you can usually apply another coat after a light cleaning and sanding.
After sealing, make maintenance a habit. Keep the bench off wet ground, tighten loose hardware, and cover it during harsh weather or store it somewhere dry for winter. If you use a cover, choose a breathable one so moisture doesn't get trapped against the wood. Once or twice a year, touch up worn spots with light sanding and a fresh coat of sealer. That stops splinters before they have a chance to form.
Using outdoor cushions on the seat and back also helps reduce direct wear and sun exposure. Just be sure not to leave wet cushions sitting on the wood for days, since trapped moisture can damage the surface. With this routine, your bench should stay smooth enough to run your hand across without worrying about a surprise splinter.