Epoxy / Polyester Resins Question

[Wotname]

The perceived wisdom is:

1. that cured polyester resin can be over-coated with epoxy resin provided the polyester resin surface is keyed, i.e. the bond is mechanical.

2. that cured epoxy resin should not be over-coated with polyester resin.

3. that cured epoxy resin can be over-coated with epoxy resin provided the surface is keyed, i.e. the bond is mechanical.

If this true, what is the reason that polyester can't be used over epoxy given that the reverse is OK?

[GordMay]

“... Andy Miller’s Conclusion
Based on Andy’s and our own testing and experience, I am confident saying WEST SYSTEM Epoxy is a suitable substrate for polyester-based material such as gelcoat. 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...”

➥ Applying Polyester Gelcoat over Epoxy | Epoxyworks

“... 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...”

➥ https://www.jamestowndistributors.co...t.do?docId=259

And an earlier CF discussion ➥ http://www.cruisersforum.com/forums/...xy-123711.html

[pcmm]

Originally Posted by Wotname

The perceived wisdom is:

1. that cured polyester resin can be over-coated with epoxy resin provided the polyester resin surface is keyed, i.e. the bond is mechanical.

2. that cured epoxy resin should not be over-coated with polyester resin.

3. that cured epoxy resin can be over-coated with epoxy resin provided the surface is keyed, i.e. the bond is mechanical.

If this true, what is the reason that polyester can't be used over epoxy given that the reverse is OK?

After the fact ALL bonds between polyester and epoxy (whether its poly over epoxy or epoxy over poly) are mechanical. have a look at the "boatworks today" youtube channel. He's debunked the myth that poly (specifically gelcoat) cannot be used over epoxy. and my experience also with gelcoat over epoxy repairs has proved to be excellent, no bonding issues. like all other repairs/overcoating you need to key the surface. I don't use any finer than 100grit when fairing my epoxy repairs in prep for gelcoat.

[Saltyhog]

The failure mode, as I understand it, is not that the gelcoat won't bond to the epoxy. It's the long term performance of that bond that is in question. Andy Miller did a simple test and proved that the gel sticks reasonable well to cured epoxy in the short term. As he overtly admitted, this is not a full on Scientific test that could be used to prove anything. It was just a way for him to try something for himself. He didn't try multiple vendors of epoxy or polyester gel. He didn't try different slight variations of mix ratio. He didn't try different curing temperatures. And the biggest most important, he didn't check the bond after 10 years in the Caribbean sun. We have it from some pretty experienced folks that it's the long term bond that's the problem. Something about the release of amines over time from the epoxy reacting with the polyester. I'd really like to be able to gel over epoxy, but I don't want to chance a big mess to deal with later if it fails. Perhaps Andy Miller will do another video some time in the future with some long term results. BTW, I really like his videos. Definitely worth watching.

[pcmm]

Originally Posted by Saltyhog

The failure mode, as I understand it, is not that the gelcoat won't bond to the epoxy. It's the long term performance of that bond that is in question. Andy Miller did a simple test and proved that the gel sticks reasonable well to cured epoxy in the short term. As he overtly admitted, this is not a full on Scientific test that could be used to prove anything. It was just a way for him to try something for himself. He didn't try multiple vendors of epoxy or polyester gel. He didn't try different slight variations of mix ratio. He didn't try different curing temperatures. And the biggest most important, he didn't check the bond after 10 years in the Caribbean sun. We have it from some pretty experienced folks that it's the long term bond that's the problem. Something about the release of amines over time from the epoxy reacting with the polyester. I'd really like to be able to gel over epoxy, but I don't want to chance a big mess to deal with later if it fails. Perhaps Andy Miller will do another video some time in the future with some long term results. BTW, I really like his videos. Definitely worth watching.

For small repairs its no worse than paint! if the gelcoat fails after 10 years that's still better performance than paint!

[lesterbutch]

Originally Posted by pcmm

For small repairs its no worse than paint! if the gelcoat fails after 10 years that's still better performance than paint!

So true! You have to wish you get close to ten years out of even the best paints.

[zstine]

If gelcoat is not good in the long term on epoxy, then what are manufactures that build epoxy boats doing? I believe Tartan yachts uses an epoxy resin in contact with gel coat... no? there must be other builders using epoxy laminates. do they all paint?

BTW, my Imron paint is 10+ years old and looks new, save a few scratches that you can't blame on the paint.

[sv.antea]

Anecdotal evidence: On my boat, I had sprayed gelcoat on top of extensive topside (West System) epoxy repairs and fairing 25 years ago and some repairs since. the finish is still in top shape except where barnacles gouged it in a docking accident. In my opinion, sprayed gelcoat is far more durable than any paint. I never waxed or polished it, but I wipe it with the bottom cleaning scruffies when I clean the bottom. There used to be a slight orange peel at first. Now it is BA smooth.
P.S., lately I had great luck with gelcoat additive called Duratex. It makes the spraying gelcoat go on without orange peel and even makes brush touch up smooth(er).

[Aeolus6]

A somewhat unrelated query about replacement caulking in old teak decks.

Replacement of polysulfide rubber caulking in newly cut rebates in a thin teak deck well supported by a non flexing underdeck with slow set vinyl ester resin or epoxy containing carbon black, finely milled cork and Cabosil.

We did this two years ago on a customers boat and it's standing up well.

Am now facing a similar situation with a 60' heavily built sailboat where the teak is thin in high traffic areas and the rubber has rolled out of the seams

In our experience, shallow rebates don't retain rubber seams well under lateral loading.

Comments ...

[Crusty Joker]

I have used Polyester on many occasions on boats, surfboards and etc. When a tech question like this comes up, I suggest you talk to an expert such as Scott Anderson at AquaTech. He makes hundreds and hundreds of surfboard of every type and lots of 'Art' projects for many people. He has left Marina del Rey area for the South Bay... He owns the rights to many labels... One of my favorite people and a great shaper! Very well connected around the world.

[NevisDog]

Originally Posted by Wotname

The perceived wisdom is:

1. that cured polyester resin can be over-coated with epoxy resin provided the polyester resin surface is keyed, i.e. the bond is mechanical.

2. that cured epoxy resin should not be over-coated with polyester resin.

3. that cured epoxy resin can be over-coated with epoxy resin provided the surface is keyed, i.e. the bond is mechanical.

If this true, what is the reason that polyester can't be used over epoxy given that the reverse is OK?

Correct, add #4: that FULLY CURED polyester resin should NOT be over-coated with polyester resin.

The reason is simple: epoxy will stick 'like sh*t to the proverbial blanket', while polyester will only develop a strong bond if applied to other resin that has not been left long enough to fully cure. Epoxy adheres well to timber or ply, for example, while polyester will tend to separate after a few years; its bond to end-grain balsa is more mechanical, from penetrating deep into the soft fibres. Sorry but that's about the extent of my knowledge.

Some very interesting stuff in previous posts about gelcoat (polyester) which I'll now read again, as that's the problem I'm faced with.

[NevisDog]

Originally Posted by zstine

If gelcoat is not good in the long term on epoxy, then what are manufactures that build epoxy boats doing? ...

Thanks for info on Imron paint.

When building new, remember the gelcoat goes into the mould first, so the epoxy is applied second. Since epoxy adheres well to polyester (or more likely vinylester in this case), but not the other way around, there are no issues in new building. Only repairs present a problem.

[boatpoker]

Originally Posted by NevisDog

Correct, add #4: that FULLY CURED polyester resin should NOT be over-coated with polyester resin.

The reason is simple: epoxy will stick 'like sh*t to the proverbial blanket', while polyester will only develop a strong bond if applied to other resin that has not been left long enough to fully cure. Epoxy adheres well to timber or ply, for example, while polyester will tend to separate after a few years; its bond to end-grain balsa is more mechanical, from penetrating deep into the soft fibres. Sorry but that's about the extent of my knowledge.

For structural bonds I completely agree. Boat builders have proven for decades that fresh polyester will not adhere to cured polyester but epoxy will if properly prepared. Some boat builders popped the hull out of the mold and let them cure before bonding the stringers or bulkheads. This is the major reason I seen so many failed tabs and stringers.

Gelcoat on epoxy (if well keyed ) will likely work well enough as the stresses are much lower than the structural bonds of items such as bulkheads.

[Lake-Effect]

My few repairs and additions to the hull and deck of our small boat were made using epoxy & glass cloth, then covered with gelcoat (topcoat). No problems yet noted with repairs, some over 7 years old. Of course, we're in freshwater, no carribean sun, 6 month season etc etc. But we do have winter which is thermal cycling.

[Saltyhog]

System Three (epoxy manufacturer) says:

Can I put polyester gelcoat over cured epoxy?

In general, polyester resins won't cure properly or bond well to epoxy resin products without a "tie-coat" barrier resin in between. System Three SB-112 resin system can be used as a tie coat in between epoxy laminating or coating resins, and polyester laminating or gelcoat resins. Using this resin allows you to gelcoat an epoxy-built or epoxy-repaired boat.

From https://www.systemthree.com/pages/faqs

From their description of SB-112:
SB-112 is specially formulated for use in building and repairing sail and surfboards over polystyrene cores. It can also be used for coating and composite laminating. SB-112 is unique in that polyester resins and gel coats may be bonded to it without the use of "tie-coats”. We recommend that polyester be bonded onto freshly sanded SB-112 epoxy within 48 hours of cure.