Restoring fibreglass

[Kevdog]

I have a 23' yacht which is around 20 years old. The fibreglass is constantly being attacked by round black growths. I have tried a pressure washer, scrubbing, bleach and all manner of cleaners but nothing prevents the return of these ugly spots which always reappear and increase in size. Can anyone suggest (a) some powerful cleaner that I may not have tried (b) some form of paint with which I can treat the fibreglass or (c) an appropriate adhesive covering? All suggestions gratefully received.

 

[Quandary]

What parts of the boat? Sounds like mildew which is like a fungus so you have to kill it with poison, I would try the good ole Patio Magic because it seems to remain effective for a long period after application but there are lots of things marketed for dealing with mildew. If bleach did not remove it, at least temporarily, I am wrong. The surface must be rough or porous to support it and if so you need to make it smooth. 20 years is young for fibreglass usually takes longer than that to give problems

 

[Kevdog]

What parts of the boat? Sounds like mildew which is like a fungus so you have to kill it with poison, I would try the good ole Patio Magic because it seems to remain effective for a long period after application but there are lots of things marketed for dealing with mildew. If bleach did not remove it, at least temporarily, I am wrong. The surface must be rough or porous to support it and if so you need to make it smooth. 20 years is young for fibreglass usually takes longer than that to give problems

Thanks for taking the trouble to reply. It's the pointy end mostly. Bleach does remove the spots but they keep coming back. I'll give Patio Magic a try. I see you are from Argyll; just had a great sail over the weekend from Largs to Rothesay and back. Thanks again.