Black stain on Caulk on gunwales and floors

[Larry B]

The caulking on Both gunwales and to a lesser degree the floor caulk have developed a dark black stain that has been damn near impossible to get off. THought was mildew but not really responding to any of the mildew cures. Don't want to put anything on the caulk that will destroy since its in good shape.
Anybody else have this issue, or any good solutions?

 

[seasick]

Without a picture, I can't be sure but assuming it is mold, once it gets into the caulk, it is very hard to remove and hard to keep from growing again. The optimal approach is to recaulk with something that has a mildicide in it.

If you want to try to remove the stain, wipe a small section with bleach using a q-tip. Get the surface wet and let it sit for a while, maybe an hour or more to see if the stain lightens.
Try to keep the bleach off of the gelcoat since bleach can cause gelcoat to yellow (permanently)

 

[Hookup1]

Picture please. Sometimes its wood in the coaming pads. Not sure what you have.

 

[SeanC]

Tried scrubbing. Tried bleach. No luck. Removed all affected caulk and re-caulking it with one containing a fungicide.

 

[SkunkBoat]

3m 4200 tends to do that. Don't use 4200.

 

[Lt.Mike]

3m 4200 tends to do that. Don't use 4200.

I used 3m5200, not sure if that’s what you meant but my boat has the same issue.
I’ve tried using bleach with some success but the black stains are stubborn.

 

[Hookup1]

I have used GE bath silicone premium mold resistant in the past. Worked well. I'm trying 3M 4000 this season. I'll let you know. Never had mine turn black.

5200 is a bad choice. It's too much of an adhesive and not mold/mildew/UV resistant. 4200 may be better but not sure. 3M 4000 is uv resistant and is similar to 4200 in that its not a strong adhesive and can be removed.