The best patching material is a styrene-based silicone sealant named Lexel. This sealant accepts paint better than other silicone sealants.
Apply Lexel to holes/cracks in decoys with a standard caulking gun (the kind that has a trigger latch that releases pressure immediately when the trigger is relaxed). Press the cartridge tip firmly against each drilled-out hole and apply Lexel, forcing some inside the decoy body to make an interior plug. Then use moistened fingertips to spread Lexel around the outside of the hole, smoothing it into a slight mound that completely covers the hole and tapers out onto the decoy’s outer surface.
Repair all drilled-out holes like this. Then set the decoy aside (out of the sun) for three days before painting, so the Lexel can harden.
Once the plugs have set, recheck the decoy for leaks that may have been missed. Do this by holding the decoy with the bottom toward your stomach, fingers on its back and thumbs on its bottom surface. Squeeze firmly. If resistance cannot be felt against the thumbs, a leak still exists. You can detect small leaks by squeezing and listening carefully or by feeling escaping air on your cheek.
Repair cracks and splits in a similar manner, except these require more drilling and filling, because the leaks are larger. First, drill holes at both ends of a crack or split. Then drill holes along both sides of the crack or split at 1-inch intervals and ¼-1/2 inch from the break. Next, after allowing the decoy to drain and dry, apply Lexel liberally into and over each hole. Then fill the crack or split with Lexel. Again, use moistened fingertips to connect and smooth all applications of Lexel into one broad patch, then set the decoy aside, so the sealant can harden.
Make major repairs, like replacing a broken bill or filling a large hole in the decoy, with a two-part epoxy putty such as Sonic Weld. (Make sure to use epoxy putty, not liquid epoxy.) Epoxy putty easily mixes and mashes out like pie crust to cover holes or serve as a bonding agent for broken-off parts.
If a decoy is leaking, but you can’t find the holes, take a close look at the plug in the injection-molding hole. This hole is usually beneath the decoy’s tail. Sometimes slow leaks develop around this hole. Daubing Lexel around the molding hole is a simple way to repair such leaks.
Once all leaks are repaired and the Lexel patches have hardened, it’s time to touch up or repaint the decoys. One option to full repainting, which is a time-consuming process, is to paint repaired decoys flat black or flat brown. These decoys certainly aren’t natural-looking, but they mix in well with decoys with regular paint jobs. Many veteran hunters prefer to have several all-black decoys mixed into their spread to enhance visibility to ducks at a long range.
There’s a certain satisfaction in shooting over decoys that you’ve restored to “good health.” Their usefulness continues, their value is retained, and their role in the grand sport of waterfowling carries on.