How do I clean the stains in my bottom paint from using Mary Kate On Off gel?

Skipper April 28, 2015 Ask The Skipper

Frank Spring asked:

I have developed black shades from the waterline bottom paint down on the stern section hull. The boat is a 2007 southport and was never bottom painted untill 2014 using blue water coppershield green ablative six bottom paint. It shows black mostly right at the waterline where I used Mary Kate on. Off gel to remove brown River water salt water stains. It worked well but the dark stains showed up recently. The rest of the hull is fine. Suggestions would be greatly appreciated. Thanks Frank Spring.

Answer:

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The anti-fouling paint contains cupric oxide as the biocide. Mary Kate On Off contains Hydrochloric Acid. The acid reacts with the copper to create copper chloride, which is water soluble. Cupric oxide is not.

It seems like the paint was damaged by the Mary Kate and the surface was washed away. The solution is to sand back the damaged paint and retouch it with fresh paint. In future, if you are going to use a strong acid based cleaner, you need to protect the paint with masking tape and plastic.

Some sailors extend the boot top to mask the waterline staining, others use VS721, which is clear, to seal the pores in the gelcoat and reduce the staining as well as making clean up easier.

Thanks for your question,

Captain Aurora
Richard Kittar