gel coat and air

If you do a gel coat repair why do you need to keep the air out while it cures. When they make a boat the gel coat is sprayed onto the mold and them is left to cure. Ture, what will be the surface of the boat is in contact with the mold and does not have direct contact with the air but the other side is side is exposed to air as it cure.

I always was told that the wax paper was to help give a finish to the surface that closely matchs to the mold as possible.

I just do not understand why it can be exposed to the air in the mold but not when repaired.

wax on, wax off
There are different types of gel coat, not all require oxygen barrier

laminating gelcoat vs. finish gelcoat

– Last Updated: Mar-08-11 3:16 AM EST –

........ laminating gelcoat (un-waxed) will cure to hard surface when that surface is deprived of oxygen (ie., the mould , sheet barrier covering , mould release spray . etc.) .

In a build process for laminating (could be a mould but not nessasarily) , the exposed inner surface of a laminating resin (in the case you are asking about - laminating gelcoat) remains tacky so the next resin/cloth to follow will achieve a Chemical bond .

When performing repairs , intially the only achievable bond is a Mechanical bond (dependent on how well you can get it to "stick") . One may want this intial resin "stick" to cure hard or remain tacky on the exposed surface . If the exposed surface is to be the finished surface , then "hard" cured is what's desired . If more build up (laminating or gelcoat buildup) is the desire , then "tacky" (for a chemical bond) is the desire .

A finish (or waxed) gelcoat will cure hard on the exposed surface because the wax in the formular rises to the surface and in that way inhibits the oxygen . The only way to achieve any further build (laminating) on top of the hard cured surface is by means of washing the wax off and sanding to prep. for a good mechanical bond again . A chemical bond is a much stronger bond than a mechanical bond .

Once the surface of a resin has cured , a chemical bond is no longer possible . The resin below the surface does cure even when the surface remains tacky . It is possible to simply wipe off the uncured tacky surface w/acetone down to the cured resin below . It is possible to cover the tacky surface (inhibit oxygen) and allow it cure hard .

OK, that makes sense.
Thanks for the explaination.

And a thorough explaination
it was! Good job.


Gelcoat chemical bond
You can achieve a chemical bond with gelcoat by bushing styrene on the gelcoat surface and then let it set briefly. I sand area and then bush styrene on the surface before using gelcoat. Thiis achieves a chemical and material bond. I also use styrene to thin my gelcoat when I need to spray it.

Perfectly true
Styrene has a very pungent odour however.