Recently, a reader asked us about the plywood backing plates for a through-hull on his boat. He said that the top laminate appeared “furry” with decay. He wondered if he could not preserve the remaining backing block with an epoxy treatment.
“Preserving” the wood with epoxy (or any “miracle” restorer for soft wood) is barely a band-aid solution. If the wood is remotely intact, the glue won’t soak beyond the surface. Also, the glue is not going to penetrate if the wood is even slightly damp, and it probably is. Truly drying wet wood requires a lot of time. And even if your epoxy penetrates beyond the first layer, it won’t add tensile strength. The wood might feel hard, but it will still snap in half.
Borate—a mixture of Borax and water—is different. It can seep into good wood and prevent future rot, because it is water-based it can soak into the cell structure. Once the rot has started, borate might slow it some, but this treatment is primarily for new wood.
If I were you, I would replace the backing plates with fiberglass. If I could not bear that, I would dry the backing plate with a fan and a heater – always attended. This can take days, or even weeks. I would then saturate the wood with a borate solution in water (about 25 percent Borax in very hot water). Brush on several heavy coats and allow to dry. Dry the wood again and then seal it with epoxy or several coats of varnish. You want a clear coat that lets you see what’s happening.
This only works for dry, exposed wood that is not significantly rotted. If you have good access, you can drill holes and inject borate deeper. This can also work with wet wood, but you will want to apply the treatment before rot damages the wood structure.
I would not use plywood for through-hull backing plates in the bilge. No matter how they are sealed, they will rot. Under the deck, they are generally okay, but I would still look at other options. For a durable backing plate, you want something that cannot rot or split.
One problem with plywood backing plates that is rarely mentioned is that they can “creep” or compress under the bolt. Once this happens, the bolts loosen, hardware moves, leaks develop and rot starts. You need to use a material that will never compress, so that the bolts do not lose tension.
The backing plate does not have to be expensive G10. Structural fiberglass is barely half the cost and still stronger than the hull. Most boatyards have a lot of fiberglass scrap lying around, so repurpose it. Alternatively, lay up four to eight layers of 1708 biaxial cloth under where the gear will be, and then drill your holes. You get a backing plate that fits perfectly, even matches the hull curves. Ounce-for-ounce it can be the strongest option. Aluminum can serve as a good backing plate under the deck, but corrosion is a problem in the bilge.
Some people advise over-drilling the holes in a plywood backing plate, filling the holes, and re-drilling. If you are going to that much trouble, just use fiberglass for a backing plate and be done with it forever.
