I have just made a mould and the customer requested a fibreglass bonnet i will be puting down polyester gelcoat and i will be infuseing.
Im thinking about using uni flow fibrrglass matt which is designed for close moulds
Btw this isnt my first post i carnt access my old account because my email got compormised old username is brainstorm and seens tho canyon is away ill have to wait to get that problem fixed 
Hi Brainstorm!
The question is?
I do not see a problem in the above.
Hi mate, question is sould i ad another layer of 300g mat behind the core so there is 2 infront and 2 behind? Or will that be enough?
That is a difficult question to answer without knowing the exact size/shape of the part or if you will be adding any additional supports like ribs or some sort of frame like a traditional metal hood (bonnet). If weight is not critical I would add another 300g mat to make the laminate symmetric which will reduce the potential for warping as the resin shrinks/cures.
Although 3 layers would be enough I always thought it best to have a balanced layup so that any shrinking is done equal on both sides of the core and should avoid warping.
I would do a balanced layup on a large flat panel like that.
A balanced laminate is not necessarily symetric and may still warp i.e. 1 layer of +/-45 fabric is a balanced laminate but will warp since the B matrix is not 0. However a symmetric laminate, which is by also balanced by default, should not warp since the coupling B matrix is 0.
What is a B matrix?
I guess it depends on how your defining balanced? 
I use the term to describe a panel that has an equal volume of fiber on both faces and a mirrored fiber orientation. This make the properties of both faces balanced.
No ribs or frame im afraid. Hopefully i can use epoxy resin to minimise the shrinkage. I will go for 4 then cheers guys
I agree that it depends on how you would define balanced.
From classical laminate theory the A, B, and D matrices describe the mechanical properties of a composite laminate. The A matrix describes the extensional stiffnesses while the D matrix describes the flexural stiffnesses. The B matrix describes the coupling between extension and flex. A symmetric laminate has a 0 B matrix which means that when it cure/shrinks (extension) it does not cause the laminate to warp (flex). A symmetric laminate (mirrored about the neutral axis) is balanced but a balanced laminate may or may not be symmetric. Just my $0.02
Ahhhh…you’re talking about a mathematical matrix and x,y,z stuff. I was thinking of “matrix” as a resin encapsulating a fiber reinforcement.
I’ll judge your $0.02 to be worth a $1.00.
I’d do 0/90, +/-45, core, +/-45, 0/90. I’d also consider adding some reinforcements/hard points around any bolting locations. Since you are using a mat (multi-axial ?), you’ll want to pay special attention to insure that the fiber alignment in each face is an exact mirror of each other. Maybe infusible will throw in another $1.