Polyester over Epoxy ?

[LONG_KEELER]

I've finished repairs on my rudder using epoxy. I'm not bothered about gelcoat on the underwater parts but gelcoat is required on other bits.

In the past I have not used polyester over epoxy but it seems that in many respects this is not true according to many on the web. Can this finally be put to bed ?

 

[greeny]

eh?

 

[thinwater]

As near as I can tell, the bond is the same as over old polyester. Not as good as in-mold where they polymerize together, but the secondary bond is sufficient if flex is minimal. If the gelcote cracked because flex was excessive, yes, it will fail again, over polyester or over epoxy. And that is probably the source of the confusion.

 

[dankilb]

Boat Work Today did a test on YouTube and it basically affirmed that, done right, you get just as good a mechanical bond as any substrate. It’s just the chemical bond that’s lacking, as above…

My vote would be: fine for gelcoat, avoid when laminating (but why would you?)

 

[Tranona]

Not sure why you want to put a gel coat on epoxy. You don't say whether you have laminated with epoxy or just coated, but if you wanted a good surface just fill and fair then paint it. best if you are laminating to use peel ply to get a good last coat finish, wash to clean then paint. You could also put a pigment in your final coat of epoxy.

 

[thinwater]

... My vote would be: fine for gelcoat, avoid when laminating (but why would you?)

I have (it's cheaper) several times, and with good technique, no problems. Only applications with a LOT of bond area. But in retrospect, I will not do it again. The bond is not as good and I know it.

 

[thinwater]

Not sure why you want to put a gel coat on epoxy. You don't say whether you have laminated with epoxy or just coated, but if you wanted a good surface just fill and fair then paint it. best if you are laminating to use peel ply to get a good last coat finish, wash to clean then paint. You could also put a pigment in your final coat of epoxy.

The reason is nearly always an underlying repair. The reason for gelcoat vs. paint is usually matching, but I've used paint with repairs and think it can be matched just as well. Gelcoat is probably more durable, but also more work to finish.

 

[Minchsailor]

Epoxy has poor resistance to UV. In industrial workit is recomended that it is overcoated with 2 pack polyreathane before the epoxy has fully cured. Could this be an option?.

 

[dankilb]

Epoxy has poor resistance to UV. In industrial workit is recomended that it is overcoated with 2 pack polyreathane before the epoxy has fully cured. Could this be an option?.

Undoubtedly this would be the best/better option, but as Thinwater suggests using gelcoat is often about matching what’s already there (or even using colour matched gelcoat, if you’re lucky enough to have some).

 

[LONG_KEELER]

Thanks for the replies.

The transom hung rudder required repairs in the underwater areas and above. I have used a mix of West 5:1 epoxy resin + microfibres which was easier for me than laminating but still obtaining strength.

I do have a supply of gelcoat for the above water parts but after hearing the replies I might just as well paint rather than gelcoat. It might even be worth painting the whole top bit.

There does seem to be some confusion that polyester resin must not be used over epoxy.

 

[coopec]

I've finished repairs on my rudder using epoxy. I'm not bothered about gelcoat on the underwater parts but gelcoat is required on other bits.

In the past I have not used polyester over epoxy but it seems that in many respects this is not true according to many on the web. Can this finally be put to bed ?

I'm not 100% sure what you mean by that.

NEVER lay-up polyester/glass over epoxy (it won't adhere)

 

[dankilb]

I'm not 100% sure what you mean by that.

NEVER lay-up polyester/glass over epoxy (it won't adhere)

Well, it probably we’ll ‘adhere’. As much as polyester will adhere to, say, a flat roof system (apparently used for this here). But it won’t chemically bond and thereby ‘laminate’. Is that what you mean?

Gelcoat is sticky stuff and sure as hell adheres to (cured, prepped) epoxy. It just adheres better to polyester!

 

[coopec]

Can you polyester over epoxy?

While epoxy adheres tenaciously to cured polyester, the reverse is not true, so the bond between an epoxy repair and an overcoat of polyester gelcoat will not be strong. Use polyester for repairs that will be gelcoated.
Polyester or Epoxy Resin? | BoatUS

YBW THREAD
Putting fibreglas resin and mat on top of epoxy resin.

 

[dankilb]

(Spoiler: The answer is ‘yes’!)

 

[Poignard]

I remade the joint between the two halves of my rudder using epoxy and woven tape but instead of trying to apply new gelcoat over the repair I just painted it white (with blue boot topping).

Below the waterline I used WEST Barrier Coat.

 

[coopec]

(Spoiler: The answer is ‘yes’!)

Here are the results.

They rely on the mechanical adhesion and they get rid of the "amine blush" . I guess that's OK for gelcoat.

But I am no expert.

I would never lay-up polyester over epoxy because I would want a chemical bond as well as a mechanical bond.

THIS IS ANOTHER PERSON'S ADVICE
This video is about Small Fiberglass & Gelcoat Repair and the dilemma of using gelcoat over epoxy on boat repairs. It is not advised to do so, but can be done with small repairs that are not structural and with perfect preparation of the area. Follow along with Brendan in the TotalBoat Workshop as he repairs a crack in the gelcoat along the deck where a track sits

 

[dankilb]

I would never lay-up polyester over epoxy because I would want a chemical bond as well as a mechanical bond.

Me too!

But I guess it might help someone caught needing to gel over an epoxy repair to know it won’t immediately fall off?!

Think the OP had come to the right conclusion re: paint. We hot coated epoxy primer on top of resin during our osmosis job and (confirmed by some amateur destructive testing beforehand) the bond was very, very strong.

 

[coopec]

Me too!

But I guess it might help someone caught needing to gel over an epoxy repair to know it won’t immediately fall off?!

Think the OP had come to the right conclusion re: paint. We hot coated epoxy primer on top of resin during our osmosis job and (confirmed by some amateur destructive testing beforehand) the bond was very, very strong.

I agree with you. It wouldn't be a major calamity if a bit of gelcoat fell off an epoxied repair.

Here is a link to a video on the subject.

This video is about Small Fiberglass & Gelcoat Repair and the dilemma of using gelcoat over epoxy on boat repairs. It is not advised to do so, but can be done with small repairs that are not structural and with perfect preparation of the area. Follow along with Brendan in the TotalBoat Workshop as he repairs a crack in the gelcoat along the deck where a track sits