glass fibre

HI, I found some sheets of glass fiber I had left over but I is there a way to tell the actual grammage of these sheets ? cutting a sample of 5x5cm and weighing it ?

I know silly I should have labelled them but didn’t, a lesson going forward !


No need to cut the tissue, it is sufficent to weight it and then measure the surface. It will be more precise then a square 5x5

Merci Claudio, make sense and so if I have 3m² of cloth and it weighs 600gr the glassfibre I have is 200g/m² right ?

Of course


I use the need to build hatch cover for the coyote I am assembling at the moment to test my strat skills as they’re rusty after years of no use. I am looking for your feedback on whether the result I got is ok or not in term of mix resine v fibre:

I use some off cuts of fibre glass cloth I had and I’m absolutely certain about their exact grammage but from my calculation based on weighing them I used 2x 163g and 1x100g and after cutting the excess I get a workable sheet of 38.7x28cm and its weight is 34g - looks good (if I have time i’ll take a quick pic to add to this post). I didn’t use any peel ply but tried to use the minimum amount of resine.

What are you thoughts on the ratio of resine vs fibre used ?


Something isn’t adding up!

First, your finished size of 38.7x28cm gives an area of approx 0.1 m2, which would indicate that each piece of the 163g cloth would weigh about 16g and the 100g cloth weighing 10g. This gives you a total cloth weight of 42g, already higher than your finished sample! I remember one of Claudio’s posts advising the amount of resin for hand layup equals the total cloth weight of the area, so in your case this gives a total weight of 82g.

With hand layup it’s not that easy to achieve 50/50 cloth & resin weight distribution - I know I’ve never done it, although from Claudio’s various build logs he seems to achieve it pretty accurately (his hulls always weigh what he predicts) without cheating and having dry (& therefore weak) laminate.

With your finished weight of 34g indicating a best possible mix of 17g resin & 17g cloth, this suggests total cloth of 170g/m2; 3x63g/m2, 1x100g/m2 + 2x35g/m2, etc etc.

At the end of the day if the sheet you’ve produced is suitable for its intended purpose then it really doesn’t matter what the cloth weights really were - out of interest, how accurate are your scales at very low weights. Although most kitchen scales indicate individual grams most are nowhere near sensitive enough to register such low levels.



Hi Row,

Thank you for your response - the one thing I know for sure is that the scale is accurate - I use a medical scale we spent €200 on so I would expect it to be very accurate.

The other element is sure is I used 2 of the heavier cloth + 1 of the lighter which was marked 100g/m².

So the final weight is accurate what may not is the grammage of the cloth I used - Need to weigh them again. But as you say it is fit for purpose as I’m gonna cut pieces of this sheet to do hatch covers for the multi I am building and for the transoms of the floats to finally close them up.

Thanks for your comments - appreciated.

Hi Row,
how is it with your back ?

About the 50/50 ratio I use for fiberglass, there is a little “trick”…

I weight first the tissue and after I mix up the resin with the same weight, but !!!
Since the tissue is cut with a certain margin (~2cm) compared to the need , once the resin is applied, not 100% of tissue surface is impregnated of course and this is why I always manage to laminate within the 50/50 ratio, to tell the full true, very often some resin remain in the bucket !
Another tip, the 50/50 criterium shall be modulated with the type of tissue. Thicker the tissues more resin is needed.
Kevlar need at least 10/12% more resin
Carbon need from my experience some 10% more resin
These ratios are the one that most manufacturers suggest in order to obtain the best strenght, this mean that adding more resin then needed the contrary is obtained

Bonsoir Claudio,
Thank you so much for your input in this discussion as this is really good information - something I can use as a guide going forward.