Originally Posted by boatster_sr4
Gelcoat is just polyester resin. You can use a thick celophane to keep the resin from air while it cures, this is called Vacuum bagging.
Vacuum bagging, doesn't really describe laying cellophane over un-cured gel to stop it getting a sticky surface.
Vacuum Bagging is exactly as it says.
For the best possible strength & lightness, the main laminate in any composite structure should have the highest fibre to resin ratio possible, as the resin is only the medium that holds & bonds all the fibres, whatever they may be, in place and to each other, too much reduces the strength and flexibility.
Vacuum bagging is the process of covering the uncured laminate in celophane, with vacuum pipes attached at various points both in the celophane itself, and also at points around the mould. A vacuum is applied to these points decreasing the pressure tween mould & celophane to a level considerably less than that of the surrounding atmospheric pressure (approx 14.5), this causes all the excess resin within the lay up to be queezed out, and compresses the laminate to a very efficient, dense structure.
Most carbon fibre structures are made this way, but are also submitted to a greater pressure from the outside by pressurising the vessel that it's cured in, which is also an oven, as in most cases, C-F is cured with the aplication of heat to the resin pre impregnated fibre matt, so no catalist.
lots of 'suck', lots of 'squash', and a bit of 'warm', and you have a wing, or a tailplane, or a race boat.
Not trying to steal the thread, just wanted to point out that VB-ing is a little more involved that just draping a bit of selotape over some gel.
PS. just realised this has already been covered, sorry to repeat.