That's an excellent question! Preventing splinters is crucial for both safety and longevity. For outdoor benches, you need a finish that penetrates deeply, flexes with the wood, and fights moisture. Here’s what I recommend:
First, always start with sanded, dry wood. For a natural, low-maintenance look, use a penetrating oil like tung oil or linseed oil. It soaks in, nourishes the wood fibers from within, and allows the grain to show. Reapply annually.
For maximum protection against rain and sun, a film-forming finish like spar urethane or marine varnish is superb. It creates a hard, waterproof shield on the surface. Apply multiple thin coats, sanding lightly between layers.
My personal favorite is a hybrid approach: a high-quality outdoor wood oil with added UV inhibitors and water repellents (like those from Ready Seal or Penofin). These penetrate deeply, repel water, and resist graying from the sun without peeling. They're easier to maintain—just clean and re-coat when water no longer beads up.
Remember, no finish lasts forever. Regular inspection and gentle cleaning are key. Avoid common film finishes like standard polyurethane outdoors, as they can crack and trap moisture, leading to more splinters. The right finish keeps your benches smooth, safe, and beautiful for years.