Stained black gelcoat

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Kendal

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Our Brig inflatable sits in fresh water from April through to October. When not in use, we have a full cover which covers everything including the tube steps. At the end of each season, there is staining on the step gelcoat which seems to be under the surface. It is only on the parts of the step which are covered (see pic). Polishing with 3m products helped to remove some of this last year but it seems to have got worse since the boat has been out of the water. Any thoughts on what is happening and how to best tackle it? I'm a bit nervous about being too aggressive with polishing products.
 

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I wonder if it could be petrol fumes, or the tubes outgassing in the heat? How well ventilated is the cover?

Just an idle thought based on very little knowledge 🤷‍♂️
 
Kendal said:
Hi Kevin, The cover is made of a waterproof material so I don't think the air passes through it.
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No vents in it, like little dormer windows? If so then my uneducated guess might not be completely bonkers - the (inevitable) fumes have nowhere to go
 
Kendal said:
I'm a bit nervous about being too aggressive with polishing products.
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I don't know what's causing that marking but you can be VERY aggressive with GRP gelcoat. In extremis, you can sand the entire gelcoat off and paint a new one on. It is infinitely repairable and replaceable. So you could use say 200-400-600-800 paper then 3M polish to remove plenty of surface material, and get to a new surface layer. The black is probably 1-2mm thick to begin with.
 
Hi, not absolutely certain this is the case with your situation (without seeing it better and the way the cover arrangement is), but it sure looks and reminds me very much of the now well known issue, that so many jet skis have suffered.
The jet skis suffer this from some less than ideal (and some cases even from their oem supplied) fitted covers which cause severe surface abrasion of the finished surface. This happens when under tow on the road. I and many others initially found this hard to believe or easy to point blame towards, but it's absolutely the cause. The cover rapidly and harshly contacts and buffs away at the finish when under way behind the car. Caused by varying aerodynamic influences during. It's also perhaps because the finish itself isn't often always a true gelcoat at all. Certainly on some modern skis and perhaps this boat too, the finish is actually a painted surface on a composite type structure or area of the craft. Or at best, a very thin coat of gel compared to the usual type found on the hull of a boat. Very common thing on modern coloured finished craft I am afraid.
Any skis I have seen this happen to (including one of mine years ago), it has been at the rear or sides of the ski that has been affected...and on a black or dark finish. The cover almost has like a vacuum/sucked tight affect and hard down onto the finished surface. Over time, this can be the affect.
Just thought I might mention it, in case it is perhaps the case here.
 
QBhoy said:
............
Just thought I might mention it, in case it is perhaps the case here.
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Zoom in and that does look like abrasion marks. I wonder if this is a combination of the two with the abraded surface now facilitating absorption of any spillage / fumes
 
are you 100% sure this is black gel coat and not painted? The white spots are unusual for coloured gelcoat. Also the chalking almost looks like it it’s under clear coat. Are the rest of the decks and console black?
 
Whopper said:
are you 100% sure this is black gel coat and not painted? The white spots are unusual for coloured gelcoat. Also the chalking almost looks like it it’s under clear coat. Are the rest of the decks and console black?
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TBH I'm not 100% sure. The hull and console are also black. I will need to contact Brig to find out. I did bite the bullet yesterday and had a shot at polishing a small area with 3M light polish on a woolen polishing mop and I'm pleased to say that it has improved so maybe the staining is just in the surface material. I'm going to have another shot with the polisher and will report back :)
 
I had similar marks on my carbon fibre car roof.
it was moisture

I used WD 40. and it cleared it without any scrubbing

WD40 was first produced as a water dispersing spray ( it’s what the WD stands for)

I then waterproofed it with silicon spray and it hasn’t come back.
 
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