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Will VS721 prevent the spread of Osmosis Blisters


Albert Sikma asked:

I bought a new 26m Macgregor sailboat last summer. I had it in the water for a couple months, and now find it has Osmosis blistering. I’m told I needed to put an antifouling paint on the boat, before I put it in the water. I didn’t think you had to worry about a few months in fresh water. Guess I was wrong. I really don’t want to do the antifouling paint. A fellow sailor told me I should use the Aurora VS721. I live in Regina, Saskatchewan. Also because I already have Osmosis blistering can I put this product on for future protection? or will these little blisters have to sanded away and a new gel coat applied…. ahhhhh… what a pain.. Anyhow any advice or help would be greatly appreciated.



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On new boats and boats that have not been affected with osmosis, VS721 has been proven to be 100% effective. It seals the pores in the gelcoat and is waterproof so it does not allow water to migrate through the gelcoat.

On boats that have already been affected by osmosis it has been proven to be effective at preventing the spread, but not always. The cause of Osmosis Blistering is complex. In most cases it’s caused by water migrating from the exterior but can also be caused by water migrating from the interior if the bilge is wet.

Resins used to manufacture fiberglass boats have changed over the years, partially due to environmental considerations, and new boats are more susceptible to this type of blistering. If water has penetrated into the fiberglass, behind the gelcoat, the blistering will probably continue, even though the surface has been sealed with VS721.

Current barrier coats are epoxy based and need to be over painted to protect the epoxy from UV damage. This is OK if you plan to paint the bottom of your boat with anti fouling paint but is not very acceptable if you want to keep your boat bottom clean and original or if you trailer or rack your boat. VS721 Bottom Coat is perfectly clear.

If you don’t have many blisters or are content to live with them it’s worth trying VS721. There is an excellent chance that it will stop the continuation. If it doesn’t work then you are faced with peeling the gelcoat, applying a barrier coat and re-gelling or painting the bottom, which is what you need to do to fix the problem.


Thanks for your question,

Captain Aurora

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