Walk down almost any block in Brooklyn or Queens and you'll see it: window frames and sills wrapped in smooth aluminum, usually white or matched to the siding. That's capping. A crew bends flat aluminum coil on site with a brake, so each piece fits a specific frame, sill, or piece of trim, then fastens and seals it over the wood.
The point is protection. Exterior wood trim needs scraping and painting every few years, and once paint fails, water gets into the grain and rot follows. Capping covers the wood completely, so it stays dry and you stop repainting.
Capping is worth it when the wood underneath is still mostly solid, or can be repaired. We check every frame before we start, cut out soft spots, and replace damaged wood so the aluminum goes over a sound surface. Capping over rot only hides the problem.
The same approach works on fascia boards and soffits along the roofline, which often take a beating from overflowing gutters. Many homeowners cap windows, fascia, and soffits together for a clean, uniform look and a roofline that keeps birds and squirrels out.