The Rot Doctor


Subject: lost on water (stringer rot?)
Date: Wed, 10 Mar 1999

I've got a 1987 Searay 36 Express Cruiser. Some how the antifreeze from my gen set is leaking into the stringer up to the port eng comp. Is the stringer in danger of rotting? If so how can I check it and repair it if possible. Also I've got some blistering on the hull, just small spots. Will auto fiberglass work or do I do something special. Last but not least what is some of the better bottom paint I can use. Thats all I can think of at the moment . Please help if you can. Boat is used on Lake Michigan. Thanks.

Let's handle the stringer question first. Stringers are commonly used as mounting points for engine mounts, and other items that need to be attached to a strong part of the boat. The usual method is to drill through the fiberglass sheath into the wood, and put a wood screw into the wood. If this is done without resealing the hole, water or any other liquid can wick into the wood and cause problems. The other possibility is that stress cracks (they can be so small as to be invisible) can allow access into the wood of the stringer. If it was just water getting into the stringer, I'd say that it is not a matter of if the wood will rot, but how long it will take. But, since you are also getting antifreeze into the wood, the antifreeze will help prevent rot in the wood. The biggest danger is that the water will migrate to a part of the wood that the antifreeze can't allowing the wood to rot there.

So that being the case, what do you do? The 100% solution is to cut the top of the fiberglass sheathing off, exposing the wood to the air, allow the wood to completely dry, then re-sealing the fiberglass sheathing. If this is not a practical or desirable, I'd find the source of the water/antifreeze getting into the wood, dry the wood at the antifreeze-entrance-point, and then seal the stringer so that the water/antifreeze can't continue to get into the stringer. If the antifreeze prevents the wood from rotting, you won't need to do any further work, and if the rot starts, you are no worse than you were before, cutting off the top of the stringer, drying the wood and applying our CPES and possibly some filler, then re-attaching the stringer top. Our CPES would be excellent for repairing any micro-cracks. It will wick into the smallest crack, and providing that there is no gap, glue them back together in one or two applications. Just be sure to remove any contaminants (oil, antifreeze) first.

Now as far as your blistering problem... The blistering is caused by moisture getting between layers of fiberglass or the fiberglass and gelcoat. The cure is to grind the blisters down, dry the fiberglass, and fill in using good quality fiberglassing materials and methods. I'd go to a marine store for supplies, most have repair kits with everything you'd need. A good library should have a book on fiberglass repair that you could refer to for techniques. Be sure to finish the repair with a coat of gelcoat, this helps prevent moisture from getting between the layers of fiberglass. Don't forget that water can enter the fiberglass from stress cracks on the interior of the boat also. BTW, one of the more common sources of stress cracks in 'glass is improper bracing of a boat that is hauled out. Always try and have the braces under or at least near to the bulkheads.

For the bottom paint, look at the percentage of toxic metals. The higher the metal content (usually copper), the better the paint will last. Generally you get what you pay for. Also talk to the dealer. They may have a formula that is especially good for the local conditions.

Any further help, please let me know.