How to clean dark spots on sika

[B4A]

Hi all,

what is the best way to get rid of these dark spots on sika junction on the deck of the boat?

I don't think Acetone is the right product, since I don't want to remove the sika, just clean it.

I've tried with soap of course, but it doesn't work.

[boatman61]

Try painting it with bleach..

[thinwater]

It's mildew. Bleach and don't expect perfection.

[B4A]

Thanks, wouldn't bleach harm the gelcoat?

[daveNZ]

In our shower at home, we roll a square of toilet paper, dip it in janola (bleach) and leave on overnight. Magic! Don’t know about long term effect on the sika though?

[GordMay]

Originally Posted by B4A

Thanks, wouldn't bleach harm the gelcoat?

No.
Bleach (sodium hypochlorite) can be mixed with water or baking soda to clean silicone caulking. If the caulking is really dirty, the mildly abrasive baking soda will help to lift stains and remove them.
To use bleach alone, mix a solution at a ratio of 1/4 cup bleach per gallon of water. Use warm water rather than hot, since hot water makes the bleach less effective.
If you want to mix it with baking soda for a deeper cleaning, fill a bowl with 1/2 cup chlorine bleach. Add 1/4 cup baking soda and stir well to make a paste.

[B4A]

Originally Posted by GordMay

No.
Bleach (sodium hypochlorite) can be mixed with water or baking soda to clean silicone caulking. If the caulking is really dirty, the mildly abrasive baking soda will help to lift stains and remove them.
To use bleach alone, mix a solution at a ratio of 1/4 cup bleach per gallon of water. Use warm water rather than hot, since hot water makes the bleach less effective.
If you want to mix it with baking soda for a deeper cleaning, fill a bowl with 1/2 cup chlorine bleach. Add 1/4 cup baking soda and stir well to make a paste.

Thank you so much! That worked remarkably well!

[B4A]

@GordMay: after cleaning the sika with the mix you recommended, next day I found these brownish stripes. Any idea why and how to get rid of them?