DIAGNOSIS The stock porcelain-coated drip tray is prone to rust where the coating chips. Acidic drippings accelerate this.
FIX Yes, you can and should use heavy-duty aluminum foil on top of the tray. But first, sand the rust off the existing tray with coarse grit sandpaper. Wipe clean. Then cover the entire tray with a single sheet of foil, pressing it smoothly into the curve. Replace this foil after every 2-3 long cooks.
IF THAT DIDN'T WORK If the tray is too far gone, order a replacement part. Consider upgrading to a stainless steel aftermarket tray designed for the 700 series—it will last longer.
HEADS UP Never let drippings pool and bake on the tray. The carbonized grease is corrosive. Clean or re-foil regularly.