I’ll be straight with you: most outdoor benches can handle rain and humidity for a while, but not all of them hold up well over multiple seasons. The difference almost always comes down to material, hardware, and finish.
Material matters most. Solid teak is one of the best in wet climates because its natural oils repel water and resist rot. It will turn silvery gray over time, but it usually will not warp or crack the way cheaper woods do. Cedar and white oak are decent too, but they benefit from a water-repellent sealer. Acacia and eucalyptus look great at first, yet in constant humidity they can swell, develop black mildew, or warp if they are not sealed regularly.
Metal frames can be strong, but they are not all equal. Aluminum with a powder-coated finish is my go-to for rainy areas because it will not rust. Wrought iron and steel can last if the coating is intact, but once you get a scratch or a chip, rust will start at that spot and spread under the finish. Stainless steel hardware also helps a lot. Cheap zinc or plain steel screws often leave rust streaks on the wood after a wet season.
Poly lumber, sometimes called HDPE or recycled plastic, is the easiest option for heavy rain and humidity. It does not absorb water, so it will not rot, warp, or feed mold. Resin wicker over an aluminum frame can also work well, but natural wicker should be kept out of constant moisture.
What actually goes wrong. With heavy rain, the first thing I look for is standing water. Benches with flat solid seats tend to hold water, which leads to peeling finish, mold, and soft wood. Slatted seats drain better and dry faster. In high humidity, the bigger enemies are mildew, black spots, and swollen joints. Over time, wood expands and contracts, and if the joinery is weak or the hardware is not rust-resistant, the bench starts to feel loose.
How to make any bench last longer. Even a weather-resistant bench does better with a few habits. Tilt the bench or wipe it down after long storms if you can. Use a breathable cover instead of plastic, because trapped moisture can cause more mildew than rain itself. Keep it off wet ground by using feet or pads. For wood, apply a teak oil or water-repellent sealer once or twice a year. For metal, touch up scratches with paint before rust gets started. Check and tighten bolts at the start of each season.
If you live somewhere with heavy rain and high humidity, I would steer you toward teak, powder-coated aluminum, or poly lumber. Avoid untreated softwood and cheap metal hardware. With the right bench and a little care, you can leave it outside all year and it will still look good.