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.

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