Yes, you absolutely can—and should—sand them smooth. I’ve dealt with this exact problem on my own bench, and it’s very fixable if you take your time. Outdoor wood splinters usually come from raised grain, weather wear, or small cracks that catch on clothing and skin. Sanding smooths those fibers down, but you also need to protect the surface afterward or the splinters can return.
Start by cleaning the bench with a stiff brush or mild soapy water and let it dry completely. I like to use 80-grit sandpaper first on obviously rough or splintered areas, then follow with 120-grit, and finish with 180-grit for a smooth, skin-friendly surface. Always sand in the direction of the wood grain, not across it. Pay close attention to the seat slats, armrests, edges, and any spots where the wood feels fuzzy or sharp. For curved or tight areas, a sanding sponge or a small piece of folded sandpaper works better than a power sander.
After sanding, wipe the bench down with a tack cloth or a damp rag to remove all dust. Then apply an exterior wood sealer, deck stain, or outdoor paint. This step is what locks the fibers down and keeps moisture from raising the grain again. If you leave the wood bare, it will feel smooth for a while, but it’s much more likely to splinter later.
If any board is badly cracked, soft, or has deep splinters that keep lifting, it may be worth replacing that piece rather than sanding endlessly. Otherwise, light sanding and a fresh coat of sealer once a year or whenever the surface feels rough will keep your bench comfortable and safe to sit on.