Before looking for the right glue, understand why the part broke. Did it succumb to a combination of UV damage, creep (distortion), fatigue, and ordinary wear and tear? In the case of UV damage, the material itself is vulnerable. If it is in an exposed location, the glue or any replacement parts should be UV-resistant, or at least protected by paint. With plastics, UV damage and fatigue builds up over the years, in the same way that chloride corrosion builds up over the years and leads to fatigue cracking in stainless steel.
Was the part simply too weak? If you repair it with adhesive, applying a generous filet can spread the load. Maybe additional material such as fiberglass can be laminated in place to add reinforcement. Consider using a polyester or nylon fabric that matches the flex characteristics of the base material. The laminating resin should be either a specialized plastic adhesive, G-Flex, or preferably a methyl methacrylate adhesive (MMA) like Plexus. Common laminating epoxies, including West Systems 105, are too stiff, will not bond well, and will peel away. If the part cracked at a bolt hole, can additional fasteners be added?
Was it an installation problem? Pumps and through-hulls are often cracked by stress on pipes, elbows, or flanges when a pipe is misaligned. To relieve this stress, reinforce these areas or replace misaligned pipe with hose. Tanks often crack due to over-torqued fittings, poor support at the sides, or insufficient allowance for expansion (plastic gasoline and diesel tanks expand due to absorption of the fuel into the plastic).
General epoxy. Most plastics are more flexible than laminating epoxies, and epoxy does not create a good bond with slippery plastics such as polyethylene and vinyl (see “Repairing Plastic Tanks,” PS December 2018).
G/Flex. Laminating epoxy flakes off many plastics, but G/Flex will bond to most plastics, even polypropylene and polyethylene if the surface is properly prepared (sanded and flame-treated). G/Flex is not UV resistant, so it must be covered or painted if exposed.
Plexus. This methyl methacrylate adhesive is the best choice for all plastics except polyethylene and polypropylene, which are better bonded with G/Flex (see “Boat Repair in the Age of Plexus,” PS July 2022). Adhesion is better than most epoxies, it cures fast and is strong. It is slightly thickened, which is helpful for reinforcing joints, but less useful if you want to incorporate reinforcing fibers. Devcon Plastic Welder (a repackaged version of Plexus MA 300) is our go-to glue for most molded plastic part repairs. Other “plastic welding” products are epoxies and have not done as well in PS testing.
Plexus is also useful when you need to join two PVC hoses without using a barbed union, which will constrict the passage. Just use a few inches of the next size larger hose to splice together the two hose ends. Look for a hose that fits snugly and has a similar wall thickness. Clean the ends and the inside of larger tube with solvent and rough it up with sandpaper. Coat the outside of the hose ends with Devcon Plastic Welder and slide them inside the larger splice piece. Within a few minutes you have a permanent flexible splice, with no reduction in inside diameter and no clunky hardware or clamps.
Loctite PL-S40. We’ve used adhesive sealant to secure baffles in polyethylene kayaks and repair polyethylene fishponds (in combination with polyester or acrylic cloth), and this repair is still holding strong 10 years later. PL-S40 is more prone to mildew than other sealants, but it adheres better to plastic.
If possible, spread the load with a wide fillet. Keep the gap between the parts you are bonding as small as possible. If you expect any flexing at the crack, use a bond-break over the crack itself, such as tape, and cover the joint with a flexible “bandage” using sealant and nylon or acrylic cloth (see “Repairing Plastic Tanks,” PS December 2018.)
Welding. A plastic welding kit can be handy for repairing polyethylene boats and tanks, because adhesives don’t bond well. Welding is technically applicable to all plastics that melt, including PVC and nylon, but results can vary wildly. Welded repairs are often ugly, but welding repairs are strong if you can melt the edges, press the two pieces together, and add filler (strips of the same plastic) reinforcement over the top. Sometimes metal screen worked into the weld can make a superior reinforcement.
