The following maybe of some interest and may help with what can /can't be used with polyester GRP or it may just be a lot of waffle!

Polyester resins are produced by condensing a glycol with an unsaturated and a saturated dicarboxylic acid. Then the polyester is blended with styrene this acts as an extender to reduce the cross linked (cured) density, thereby reducing the brittleness of the cured resin. An accelerator is also added, with the catalyst mixed in immediately before use.

The ester link within the polymer chain is chemically the weak point, being susceptible to hydrolysis, ammonolysis i.e. attack from water / ammonia or other strong alkalis, this can lead to chain scission and a breakdown of the resin structure and ester interchange. The polar nature of the ester link may also act as a proton acceptor allowing interaction with other groups.

I cannot find out what the 'rust inhibiting surfactant' is but most surfactants are polar - they are 'soaps' used to breakdown surface tension. To 'kill' rust I would think that is likely to be an alkali to increase the pH. So my feeling would be that waxoyl could weaken the surface of the GRP to allow a strong alkali to attack the polymer chain.

These are only my thoughts but may help with making a decision.

To finish a bit on osmosis.

All laminates will permit very low quantities of water to pass through in vapour form. As this water passes through it reacts with any hydrolysable (ester group or other contaminates) components within the laminate to form cells of concentrated solution. Under the osmotic cycle more water is then drawn in to attempt to dilute this solution. This water increases the fluid pressure within the cell to as much as 700 psi, eventually causing the laminate to burst giving a 'chicken pox' effect to the surface. The areas of attack will normally be exposed fibre ends and damaged gel coat.

This answer was provided by Ian Cummings (BEng Polymer Technologies)

Footnote: The manufacturers of Waxoyl, Hammerite Products Limited, recommend against using Waxoyl on or near fibreglass products.