Rust near sprinklers and fountains usually comes down to three things: constant moisture, splash and overspray, and water chemistry. I’d tackle all three rather than relying on one magic coating. Start with the furniture itself: for exposed urban settings, choose 304 stainless steel, or 316 stainless if there is chlorinated fountain water or salt nearby, plus powder-coated aluminum, galvanized steel with a well-sealed finish, or other rust-resistant materials. Avoid cheap mild steel hardware, and check hinges, screws, and welds, because those often rust first.
If you already have metal furniture, keep the finish intact. Wash off fountain or sprinkler residue with fresh water, dry it with a cloth, and apply a suitable wax or sealant for the material. Touch up chips and scratches promptly with matching paint or a rust-inhibiting primer. Keep legs out of standing water by using rubber feet, plastic glides, or small risers, and make sure the area drains well. If possible, adjust sprinkler heads so they do not hit the furniture, and move pieces a little farther from fountain spray zones. Breathable covers help during off-season or heavy overspray, but do not wrap furniture tightly when it is wet, because trapped moisture makes things worse. In fountain areas, rinse more often, since chlorine, minerals, and salts can speed corrosion, and avoid harsh bleach-based cleaners on metal. A quick weekly wipe-down and a monthly inspection of joints and scratches will save a lot of grief. If rust has started, remove it with a fine abrasive, treat the spot, and reseal; do not just paint over it. In short: choose the right metal, reduce water contact, promote drainage and airflow, and stay on top of maintenance.