Thin gel coat?

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RobBadgie

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Hi, there were a few chips in my gel coat that I filled and sanded with wet and dry paper. All nice and smooth now. However, there are dark marks showing through the white gel coat. Its like there is a thin white film of paint/colouring over dark fibreglass and I've just sanded that off. Any idea what is happening and how I can make it white again? Thank you in advance!
 
Sounds like you've neerly gone through the gell coat. Did the same once. If you want to fix it you will need to add a layer of gelcoat on top.
No easy a probably best left to a pro. If you do have a go I think spraying would be the best option to minimize the sanding. Problem I always have is fading the old and new gelcoat together. Still cant figure out how the pro's do this.
 
RobBadgie said:
Hi, there were a few chips in my gel coat that I filled and sanded with wet and dry paper. All nice and smooth now. However, there are dark marks showing through the white gel coat. Its like there is a thin white film of paint/colouring over dark fibreglass and I've just sanded that off. Any idea what is happening and how I can make it white again? Thank you in advance!
Click to expand...

can you get a picture of the area?
 
I can this weekend, thanks. The odd thing is that it doesn't look like the gel coat (which is on deck) is worn down at all.
 
did you sand it down before filling, and by how much?
is the dark area around the filled area?

hard for me to say so just asking to make a picture in my head.
 
rlw said:
No easy a probably best left to a pro. If you do have a go I think spraying would be the best option to minimize the sanding. Problem I always have is fading the old and new gelcoat together. Still cant figure out how the pro's do this.
Click to expand...

They mix pigment in until the colour matches up. The stuff doesn't change colour while curing. Look up "Boatworks today" on YouTube for examples of this (and lots of other excellent advice).
 
There are a few other tricks to match gelcoat as well. (and frankly a few are staying under my hat)
However don't try and match the oxidised surface colour for a start, which will have a yellowing tinge even on a boat thats two or three years old and it can vary between the top and bottom of the topsides.
You need to cut back the area with 400grit to get the correct colour to match.
The repair needs to taper and go so thin that it turns opaque so you can't see the join.
To do this you need to taper the sanding. (pro tip)
So imagine a 1" hole that's been repaired.
The outside diameter of the gelcoat would be around 6 to 8"
The outer would be keyed with 120 to 150 grit and increasing down to 40 or 60 grit in the center.
I'd apply 4 thin coats of gel and one with gel with wax or top/flow coat.
Sand back initially with 320 then 600 then 1000 and lastly 2000grit.
Then I use a medium/fine cutting compound and buff.
Job's a good-un.

If your gelcoat is thin then its really the same trick.
Do not be tempted to apply heavy coats as you will get tiny air bubbles that will show up once you get to the cutting stage.
On a warmish day I can usually get a layer ready for the next coat in an hour or so.
 
Gelcoat is not the best coating to use to repair dings. Yes it is the original material but that was applied and allowed to go to gel stage (not hard) before the GRP was applied. So ideal for original application but not for subsequent adding.
Something like a polyurethane 2 pack paint is best for later application. You still get the problem of getting the colour right and might need to experiment to exact colour with pigments. Despite these facts about gel coat people still use them for repair and generally it sticks and does an adequate job. But still not the best olewill
 
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