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Fibreglass Lay-up - Time Window Between Successive Layers?

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I'm about to lay up a mould of around 2m2 with polyester gelcoat and resin/CSM layers by hand.  I've successfully laid up smaller sample areas, but I am concerned as to whether I will be able to work fast enough to get successive layers on before the layer underneath gels/cures too much. Could anyone advise as to how far cured the resin can be and still chemically bond to new resin? Does the resin still need to be in the gel phase, or is there a window of hours after it hardens? On the same subject, when hulls are laid up, is it a continuous job, or is it possible to break over night between layers? 



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37 minutes ago, BB37 said:

Does the resin still need to be in the gel phase, or is there a window of hours after it hardens?

I would think there is a window of several years. Otherwise, how will later damage repairs still bond successfully with the hull? Or how would you later be able to glass in a bulkhead onto the inside?

I hope that a trained laminator (which I am not) will come and give more technical detail, but I would say it also depends on the ratio of hardener with the resin. Not very much and it will take a long time to go off, but if you give it a "hot" mix it will harden in minutes. Trouble is it may then also catch fire!

You can also do a hot mix with P38 fibreglass filler, when it will then go off underwater, if you are trying to stop a boat from sinking.

You will need 2 coats of gel, maybe 3, depending on its consistency. When laying up the matt I would be most concerned to make sure you have enough resin, and then even more resin, so that you fully soak the matt each time and work it well into the strands. Do you have a small laminator's roller? Very useful. An old friend always told me that you can never use too much resin.

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The resin and csm need to be applied to the gelcoat within a few hours and certainly within 24 hours. three layers of 600 gram chopped strand matt and resin will give a laminate thickness of approx. 5mm thats about as much as you want to laminate in one go. If you apply more layers than that there can be an excessive heat build up. East Coast fibre glass supplies have a good website with good advice on layup procedures.


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If you have let the glassfibre  / resin go off it's advisable to wipe down / clean the surface with acetone before putting the next layer. As some resins have a wax in them which floats to the surface, and may prevent bonding of the next layer.

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  • 2 weeks later...

Catalyst hardener should be added at between 1 and 2% according to air temperature. This is best done by weight as it's more accurate. Too little hardener will result in the resin not curing to its maximum strength whereas too much will mean the final result is more brittle and potentially likely to fracture.

After coating on your gel coat it would be normal to apply a single layer 300g matt/resin as soon as the gelcoat is cured. After that you can apply up to 3x 600g layers in one go being aware of too much build up in corners etc. Like Vaughan said, in theory you can wait years to apply more layers, the only issue being cleanliness of the job preventing proper adhesion after a period in storage. 

If a job has become dirty or dusty a good sand down will get you back to something worth laminating to!


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