STRONGBACK DISCUSSION etc.

[Scott Galloway]

Well you must mix up a better batch of epoxy that I ever have. Also my recollection is that I have read in some marine publication that the bond between epoxy and gelcoat is not a secure one. Can't say that I recall where I read that exactly. The previous owner of my boat drilled a few holes and filled them with epoxy. Gelcoat stuck to that for awhile. I removed a spinnaker pole holder and filled those holes with epoxy as well and ditto for the failure of those bonds in time.

I mixed my batches of epoxy as per the West System's instructions. All of the epoxy that I have covered with gelcoat was cured and sanded before the application of the gelcoat, but I have no idea at this point what the ratio of resin to catalyst was, since that was a number of years ago. The epoxied areas were very small in relation to the areas to which I applied the gelcoat. If it works for you, that's cool, but my few experiences with gelcoat to epoxy bonds have not been 100% positive.

The reason that I use gelcoat is that my boat has, as did all Ariels, a gelcoat skin on top of the fiberglass deck molding. I applied a new coat of gelcoat in several areas where the original coat was worn or where holes had been drilled and filled, or where stress or impact cracks were evident. I like gelcoat. It's pretty stuff if you apply it correctly, and it looks and lasts better than LPU. As to epoxy being a safe product to use, the safety of all marine products in my opinion is a matter of mythology, unless of course you are applying steamed white rice to nori, in which case only the raw fish that you apply afterwards is potentially toxic.

[ebb]

It's a bit of a trick. When a polyester/glass boat gets laminated they spray gelcoat into the female mold and after partial set continue with the layers of glass and plastic. Can be said that the gelcoat in this case has a CHEMICAL bond with the laminate.

In the case of most repair, restoration, or new added work, gelcoat is the last layer. Most will distinguish this type of layering as MECHANICAL. And the gelcoat is going on as a coating. That's why a DIY looks for the most versatile product that is made for the job. I have heard that cured polyester can be 'softened' with styrene or acetone to help get a 'chemical' bond with a new polyester layer. That would include polyester gelcoat, I guess. Who wants to get into that!!??

We can think of ANY layer or coating as only achieving 'mechanical' bond - like a primer coat or finish paint. These days the most common and probably the best primer/sealers are epoxy based: single or two part, solvent or waterborne. A modern gelcoat product no doubt takes that into account. Paints and coatings can be almost any formulation these days including polyester. The epoxy undercoat usually sticks better and seals and neutralizes surfaces better.

Used on the exterior: gelcoat will protect UV sensitive epoxy. Gelcoat won't stick as good as epoxy, so prep is important.
These days every coating is modified one way or another: powder coating is done with modified polyester that produces good adhesion as a long lasting baked-on coating - usually on metal.