The Rot Doctor


Subject: mildew problem
Date: Mon, 10 Sep 2012

Hi Rot Doc,

I have a 25 year old carvel planked schooner we built and have kept here in Belize. We have used good quality oil based enamels fairly successfully through the years. In recent times we are plagued with grey mildew working its way through the paint a few months after coating, particularly in the lower part of the topsides (pitch pine). I have sanded back to clean paint or bare wood and tried using Gluvit as a sealer and over coating with 3 coats of enamel. Results were no better and even some blistering has occurred. Possibly I haven't given the epoxy enough time to cure? (done in the boatyard hustle) I am wondering if polyurethane enamels would do better but figure they are too rigid for a carvel hull. The mildew is wide spread and we have gotten fair results on the rails where we burned the paint off. Maybe we killed a few of the mildew mites. Any suggestions?

It doesn't take much additional moisture to push wood up over the minimum threshold for wood rot and mildews to start when you are in humid environments like Belize. It would be tough for me to pinpoint the exact cause, but my guess would be too much moisture in the wood. Personally I would try a few things. The first would be to see about increasing air circulation within the boat. When not in use, leave lockers and bilges open. Try not to close the boat tightly. Provide active ventilation if practical. This should help the evaporation rate of moisture from the inside of the hull. Second, chase down any rainwater leaks. Even a small leak could increase the amount of moisture in the boat to above the threshold of rot and mildew. Third, the next time you sand the hull bare, soak some borates into the wood. Borates are a natural and cheap fungicide/insecticide and should help control the mildew spores, even if you can't keep the wood dry.