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Post Info TOPIC: Silicone turns fibreglass yellow
Mamil


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Silicone turns fibreglass yellow
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I'm replacing the side/running lights on my van which is constructed of polyester fiberglass coat on plywood. When I took the old lights off I noticed that the white gel coat of the fibreglass had gone yellow under the lights, presumably because of some chemical reaction with the sealant used.

 

Here's one I put on there about a year ago and used standard neutral cure white silicone.

Yellow stain 1.jpg

 

And here's one that was original from the factory, and only has very slight yellowing even though it's been on there longer. I don't know what sealant they used.

Yellow stain 2.jpg

Has anyone else experienced this yellowing? Can anyone suggest what's the best sealant to use so that it doesn't happen - maybe a polyurethane based one instead of a silicone based one?

 

I know you're probably thinking - it's under the new light, what's the problem? Well this time it is, but there's other times I won't be covering over a old fitting so want to know how to prevent this happening in future!

Thanks.

 



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Greg 1


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Part of the issue is that silicone is not compatible with plastics be it acetic or neutral cure. Use a polyurethane sealant adhesive. For what you are doing, Sika Pro is good and readily available at the big green shed. A side benefit is that the dirt doesn't stick to urethane as badly as silicone and it is paintable.

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Greg O'Brien
Mamil


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Greg 1 wrote:

Part of the issue is that silicone is not compatible with plastics be it acetic or neutral cure. Use a polyurethane sealant adhesive. For what you are doing, Sika Pro is good and readily available at the big green shed. A side benefit is that the dirt doesn't stick to urethane as badly as silicone and it is paintable.

 Thanks Greg, off to the Big Green Shed to get some Polyurethane based sealant!



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El Gringo


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Mamil wrote:
Greg 1 wrote:

Part of the issue is that silicone is not compatible with plastics be it acetic or neutral cure. Use a polyurethane sealant adhesive. For what you are doing, Sika Pro is good and readily available at the big green shed. A side benefit is that the dirt doesn't stick to urethane as badly as silicone and it is paintable.

 Thanks Greg, off to the Big Green Shed to get some Polyurethane based sealant!

 

Hope you never want to get it off again???

I have never seen neutral cure (roof and gutter) silicone do that, but maybe i live a sheltered life - done lots of roofs but...

 

Cheers,



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Peter_n_Margaret


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Hope you never want to get it off again???

Easy with a multitool.

Cheers,

Peter



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Mamil


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El Gringo wrote:
Mamil wrote:
Greg 1 wrote:

Part of the issue is that silicone is not compatible with plastics be it acetic or neutral cure. Use a polyurethane sealant adhesive. For what you are doing, Sika Pro is good and readily available at the big green shed. A side benefit is that the dirt doesn't stick to urethane as badly as silicone and it is paintable.

 Thanks Greg, off to the Big Green Shed to get some Polyurethane based sealant!

 

Hope you never want to get it off again???

I have never seen neutral cure (roof and gutter) silicone do that, but maybe i live a sheltered life - done lots of roofs but...

 

Cheers,

Yes, first time I've noticed it happening, and I've used silicone on boats for years, but then thinking back on it maybe I didn't remove many things again to see what it was like under there?

Agree it is more difficult to get polyurethane off, but I remember a caravan repairer told me the trick was to soften it first - I think he used turps, but not sure, I'll have to ask him again.



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dieseltojo


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Nothing wrong with a bit of silicone in that situation at all. But you need to remove it all as silicone will not stick to a silicone under base. That is,if you are worried.

You see, as long as the silicone is there even with out sticking, it is in effect a gasket to keep water out. It will do that even applied over that silicone.

We will never know what silicone was used initially, but some are not UV proof maybe or that lamp made a bit too much heat on occasion.   Just saying.smile

I would be putting some sealant in the actual hole where the wire comes from in any case.



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