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Osmosis Protection for Boat Bottoms


Rob Gropp asked:

How many coats of VS721 would you recommend to start with on new gelcoat to give a seasons protection on a hull that stays in the water for extended periods of time. This is a freshwater application on Georgian Bay. I am most concerned with inhibiting osmosis. Any specific information you can provide will be appreciated. I would also be interested in knowing how hull cleaners that have muriatic acid as a component will affect this product.



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Two coats of VS721 will be adequate to seal and protect your boat bottom from osmosis blisters. Companies like Larson Boats have been recommending this product for over 10 years and it has eliminated osmosis from their boats. Be sure that you prep the hull with Boat Scrub before applying the VS721.

VS721 is resistant to strong acids such as muriatic, but bottom cleaners can have other ingredients which can break the polymer. VS721 needs to be re-applied at the beginning of each season so this should not be of concern. If you use your boat on a regular basis to keep the slime from building up or wipe the slime off the bottom periodically, your boat should be fairly clean at the end of the season so you will not need muriatic acid. If you do let some slime accumulate, it will wash off with a solution of Boat Clean Plus and water.

Be careful when using muriatic acid as it can pull the plasticizers out of the gelcoat which can result in stress cracks below the waterline. A good bottom cleaner will be inhibited so that it does not attack the gelcoat.

Many boat owners apply one coat of VS721 in the fall and a second in the spring just prior to launch to save commissioning time in the spring.

Thanks for your question,

Captain Aurora

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