Originally Posted by David Tilston
It should be noted that while epoxy itself may be stronger, then polyester resin, most of the strength is in the glass. Many people from a boating background recommend epoxy. But there are problems with epoxy. Mainly the cost, and that only epoxy sticks to it. Our trailers were built with polyester resin, and that is what I use when doing fibreglass work. Epoxy is just not necessary.
I must say that the main problem is that polyester resin has a very poor adhesive power over cured fiberglass It is strictly a mechanical, not a chemical bounding. It must also be understood that waxless gelcoat would not cure in presence of air, so waxless gelcoat is initially sprayed inside the mold on new shells then fiberglass layers are applied on gelcoat that is not completely cured, to create a chemical bound. Once the fiberglass is cured, gelcoat will not create a chemical bound anymore, but it only relies on mechanical bound. Note that on polyester or epoxy, the result is the same.
The other consideration is that Inside the original mold, wax has been applied. On cured fiberglass repairs (polyester or epoxy) only waxed gelcoat can be used so any repair over gelcoat will likely delaminate under stress because of the absence of a chemical bound and the wax contamination from waxed gelcoat. Sanding gelcoat will not eliminate wax, but it will contaminate the micro pits and compromise the mechanical binding we try to create with sanding.
In short, no matter what repair you want to do, a structural repair that will withstand vibration requires to remove the wax residue with a product such as Interlux solvent wash 202 (much better than acetone which evaporates too quicly), sand to create a good mechanical binding and preferably use epoxy for critical and structural repairs because it will chemically bind to cured fiberglass.
As for the myth that gelcoat cannot be applied over epoxy, please check the following link: WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Polyester over epoxy
Our test clearly demonstrates that polyester gelcoat will bond to a properly prepared WEST SYSTEM epoxy as well as to a cured polyester laminate. This test confirms that polyester gelcoats can be applied over cured WEST SYSTEM ™ epoxy on repairs below the waterline.
Of course, I must say that it is true that delamination has more chances to occur under the waterline than on an RV door hinge mount. Delamination on boats is caused with the fact gelcoat is not 100% waterproof and that water may cause delamination over time. On an RV, let's say our only concern is a strong repair that will last. Experience proved that the original hinge mount design does fail over time, so why use the same material over again ? Why hesitate for epoxy considering the advantages: (see http://www.amtcomposites.co.za/sites...0Polyester.pdf
■ Better adhesive properties (the ability to bond to the reinforcement or core)
■ Superior mechanical properties (particularly strength and stiffness)
■ Improved resistance to fatigue and micro cracking
I think the above advantages of epoxy are still worth the extra cost for someone who will perform the repair on his own and wants to make sure the repair will last with minimal technical experience. For instance, Interlux watertite is a 50/50 mix (easy calculation), as opposed to polyester resin hardener dosage.
Considering the small amount of epoxy that needs to be purchased for resetting hinges mount, it is worth every extra dollar in my humble opinion, but I understand that an experience pro will probably do a good enough job with polyester resin repair by avoiding most mistakes a handy man may overlook.
By the way, I'm not a professional, but I have the bad habit to read instruction manuals and technical specs prior to use anything.