Hi, I am planning on doing a carbon covering of the ugly fibreglass hull (inside engine bay not exterior) of a jet ski. It will be purely cosmetic with the green 'decorative' carbon sheeting. Ive calculated I will need around 2m of the carbon, and wanted to know firstly roughly how much resin I would need to mix, and secondly if it would be ok to use a coat of resin on the fibreglass first and allow it to 'tack off' to help keep the carbon in position before a thick coating of resin.

Totally fine to put a coat of resin on the fibreglass and let it tack off, in fact that is the proper method for skinning. 

Hanaldo; I think you are making a miscalculation if you use the full 2sqm and you count a perfect 50/50 resin/fiber ratio on a 200g/sqm you will need 

Wasn't a miscalculation, I was just using a 60/40 fibre/resin ratio which is what I normally aim for with infusion. You won't get that with wet lay unless you are very good. 

You describe it as "ugly fibreglass" so I assume we are talking about a B side finish you are wanting to cover. It is probably just wet laid CSM so wont be particularly flat. I would be going over the surface with 80's grit paper to remove any high spots and key the surface. As its the engine bay there may fuel/oil contamination so a good degreasing with acetone will be needed.
The hull has probably been made using polyester resin. Polyester doesn't bond to polyester particularly well. You don't mention what resin type you are planning to use but I would suggest epoxy.
The coloured cloths tend to have a slightly higher resin uptake so I would increase your resin usage calculations slightly. They are also more fragile. The fibres come apart easily. If you are using a resin tack layer you will need to be very careful if you have to lift and reposition the cloth during lay up.
Depending on the colour of the surface you are skinning you may find it shows through the coloured cloth affecting the surface finish. One layer may not be enough. It is normal to pigment the tack layer black when skinning but this may show through the green/carbon cloth you want to use.
Warren

Carbon Copies Ltd

If it is a glassfibre B-side with chopped strand- then to ensure good bonding, you should sand it down thoroughly (it does not need to be perfectly flat) then use a clean wire brush to key it as well (that will get into the low spots as well)

Then use a coating epoxy, or thicken a laminating epoxy a bit with a filler.. you can colour the base-coat a dark green to match the green diolen in the black/green CF-diolen decorative cloth.   That works just fine...

Cure it to tack, then lay up the fabric ... then brush and bristle roller on laminating resin.

Good luck ...

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