Laying fibreglass

[Sergy]

Laying fibreglass
Hi I am going to lay 4 layers of mat over a 8” by 6 wide area on my Skeg in order to stop water from entering. Can someone advise if I start with the smallest cut mat and work my way up to 8X6 or do I start with 8X6 first and work my way down

[tandem tantrum]

I would think to go small to large with the last one going on a well sanded to shape final layer.

[motion30]

what t t said as you have ground out the spot with a taper

[minaret]

For matt it really doesn't matter.

[tandem tantrum]

True because matt is all short unidirectional pieces any way. I was thinking weave.

[minaret]

Yea you're right, with a weave in a tapered repair you want to start with the small piece. I've known some guys even in the boatyards who insist on the opposite, but it never made sense to me. If you start with the small piece and work up, then each piece mates to the original surface of the repair around it's edge as you go. If you start with the big piece first everything else you put on after that relies on the bond of the first piece. Plus if the repair has any depth to it it makes for a wierd misalignment of the fibers in the laminate.

[roverhi]

Large and work down. That way, any grinding still gives the glass total coverage of the puka. If you put the largest piece last, you could grind away this overall piece fairing the puka. With matt it doesn't make any difference as it's just short fibers.

[dohenyboy]

West System video says lay down the largest first

[minaret]

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Good fiber alignment in repair. Small piece first.


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Bad fiber alignment in repair. You put the big piece first and all the fibers in your repair laminate have to follow the angle of your grind across the depth of the repair. Instead of being in the same plane as the fibers in the original laminate, they are at an angle to it. When the laminate flexes the forces will be unevenly distributed. When cured in cross section instead of looking like a series of flat plates on top of each other, each ovelapping the other and in the same plane as the original material, you will get what looks like a series of bowls nested inside each other. It's not as strong or as easy to fair that way.
I'm guessing the WEST guide is saying to start with big piece first on a fresh structural laminate, not a ground out repair, which is correct also from a fiber alignment viewpoint.
Sure is tough to try to make a pencil sketch here...

[Sam Plan B]

You didn't mention if you are going over an edge of the skeg. Matt does not do that very well. You may be better advised to use four layers of fiberglass cloth. Regardless, rough sand the existing GRP surface about 6" back onto the good surface. Over lap the cloth about 2" with each layup going from new fiberglass to old. It works like a scarf would in wood construction. There is no reason why you could not use epoxy resin if you use glass cloth. It has better adhesion and a longer working life. It is also more expensive than polyesther by a facrtor of 2.5x. You will find it very hard to use matt with epoxy resin because the matt is held together with a binder that disolves in polyesther. But, as I said, you will find it hard to go around a corner with matt anyway. You should consider buying a copy of Hugo Du Plessis' book, Fiberglass Boats . You might find a copy on ebay or on abebooks.com. It tells all there is to know about GRP.

[Sea Horse]

I have and still use West System on repairing hole using a 12 to 1 angle and starting with large piece first working out to the small one. No more then 4 layer at a time cover with a field ply. I have had good luck with this and repaired some big holes. So that my 2 cents, and it work for me. Good luck

[SkiprJohn]

I've always used the alternating of mat, roving, mat, roving and cloth for the last layer. Always put the small piece in the hole first and start building up to your last layer of cloth that can easily be faired. I use epoxy. Its always worked well for me.
kind regards,

[Pete R]

Originally Posted by SkiprJohn

I've always used the alternating of mat, roving, mat, roving and cloth for the last layer. Always put the small piece in the hole first and start building up to your last layer of cloth that can easily be faired. I use epoxy. Its always worked well for me.
kind regards,

This, but mind that if you lay an epoxy patch over polyester, it can be difficult to fair the surface, because the epoxy will be more resistant to abrasion.

[s/v Jedi]

OP didn't say if he is gonna use polyester or epoxy;

if polyester: I have no clue, never used it.

but for epoxy, do not use mat, just cloth and woven roving, and start with the smallest piece. For reasons as to why, check the literature.

ciao!
Nick.

[smj]

I was taught from the boat builder I worked for to start with the big piece and work your way down. Same reason as stated above, when fairing you don't risk grinding through your largest piece. And always use epoxy when repairing over polyester.