Gelcoat over EPOXY

[kcon]

Here we go!


Despite what I've read here, and elsewhere, time and time again I have found various research suggesting that using gelcoat over West System 105 Epoxy, that "gelcoat-to-epoxy adhesion will be as reliable, if not better than, a polyester-to-polyester adhesion, providing proper steps are followed throughout the repair process."

I'm sure most of you are familiar with Andy Miller of Boatworks Today on YouTube.

Follow up video:

So here's Andy's post regarding his above video: http://boatworkstoday.com/archives/1668

Here's an informative post based on the information from Andy's video: http://epoxyworks.com/index.php/appl...at-over-epoxy/




Another Source, here's a write up that I've seen quoted on several sites such as JD and West System.
http://bit.ly/2yp24XN




I found the following quote on the site "cruisersforum" from an old post

Q. I have heard that gel coat will not adhere well to epoxy. Can you confirm this?

A. There is a common misconception, fueled by some gelcoat manufacturers and by some expensive failures in the field, that gelcoat will not bond to epoxy. Polyester resin bonds poorly in a secondary (mechanical) bonding situation which consequently makes epoxy the resin of choice for repairs. How can one be squared with the other?

The answer is surprisingly simple - gelcoat does bond to a properly cured and prepared epoxy surface. There are a couple issues to be aware of to have success making this repair. There are three situations that cause gelcoat to not cure over epoxy... all related to the hardener chemistry. Epoxy hardeners are basically a blend of amines, which can terminate the chain reaction of the radical molecule that is the basis of polyester (and vinylester) cure chemistry. So by carefully mixing, curing, and preparing of the epoxy so that there are no unreacted amines to interfere with the gelcoat cure, gelcoat bonds quite well to epoxy.

The first situation is undercured epoxy. Gelcoat applied to undercured epoxy will be in contact with unreacted amines and the cure will be halted.

The second situation is if the epoxy is mixed off ratio so that it is hardener rich, again leaving unreacted amines free to interfere.

Third is the issue of amine blush, commonly called blush. Blush is a surface phenomena that is a reaction of the amine molecules at the surface with the carbon dioxide in the air. It forms easiest in the presence of moisture, so working in cool, humid environments will maximize the formation of blush. Any amine hardener has the potential to blush, but it can be minimized by careful choices of amines in the formulation. In fact, WEST SYSTEM 207 Special coating hardener is one of the lowest blushing hardeners on the market and still maintains structural properties on par with our other hardeners. Regardless of chemistry, blush is very easily dealt with because it is water soluble. A simple wash with clear water removes the blush. No soap, no solvents. Then sand that washed surface with 80 grit paper to provide the gelcoat with sufficient key so it won't run. Be sure to use non-air inhibited gelcoat that has a paraffin wax added. Gelcoat is applied over epoxy on a routine basis everyday in boatyards that are aware of these issues.
Bruce Niederer, Technical Services West System Inc.

I've seen this discussed here in the past, but with only negligible amounts of information as to why, such as "poly won't bond to epoxy". So I wanted to kick up a discussion and see what you guys, most of which have far far more knowledge and experience on the matter than I do, have to say about it. I did this research because interested in using gelcoat on my project, at least for the topside. I'm most likely going to be painting but I wanted to consider all options.

With the information I've found I'd be perfectly comfortable doing a repair on a gelcoat boat with West System Epoxy, but what about doing a full boat with fresh gelcoat over dozens of epoxy repairs and fairing? What if I sealed a deck in with epoxy but I still want a nonskid surface?

 

[zool]

As you posted above, you will most likely be fine with gel over blush free epoxy repairs, but, trying to re-gel an entire boat and have it cosmetically smooth, is another story....outside of a mold, getting gel smooth is a daunting task....thats why most boat restorations are finished with an epoxy primer under a linear poly, acrylic urethane, or enamel..

Making it non skid is basically the same with paint or gel.

 

[Woodonglass]

The one flaw I see in the quoted post isPoly..." Polyester resin bonds poorly in a secondary (mechanical) bonding situation which consequently makes epoxy the resin of choice for repairs. How can one be squared with the other?"
Polyester resin forms a very good bond to old Polyresin if it's prepared correctly. Gelcoat IS a Polyester based product so this is the Main Reason to do the repairs to your Polyester Resin Hull. Gelcoat adheres very well to for a Total Hull recoating. Epoxy can be compatible with Gelcoat but there's too many variables for me to be a fan of using it. It's always a safe bet to keep similar products together. ALL the epoxy boys will say that epoxy is the way to go but Poly has been used for a LOT of years and I've never heard, nor seen, a failure unless it was poorly applied. Even epoxy will fail if not applied properly.

 

[zool]

What they are saying Mike is once the chemical bond between the poly substrate and topcoat window has diminished, meaning completely cured, Epoxy will bond better than poly to a sanded substrate, which is true......this still doesn't address poly based gel over fully cured epoxy as opposed to fully cured poly.

 

[ondarvr]

I sell polyester products that are compatible with epoxy, but we don't say they will work with every epoxy because there are just too many formulas out there.

What I have seen over the last 45+ years is that while it can work, it doesn't always work, it can depend on the exact gel coat and the exact epoxy.

I almost forgot to add that I sell epoxy too

 

[Scott Danforth]

you also forgot to add you sell the stuff that makes it into boats...... and molds.....and tooling used for boats......