Putting fibreglas resin and mat on top of epoxy resin.

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Pete735

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I've added an additional vertical floor support from the bilge up by using woven mat and epoxy resin to give the joint strength - fibreglas hull. This has been done after using 40grit disc on an angle grinder and acetone to clean area prior to applying epoxy and woven mat. 2 layers of mat applied along its length.

Any reason why I should not add some bulk to this by using chopped strand mat and fibreglass resin instead of epoxy. Reason is I've got loads of csm and fibreglass resin left from a previous job. I was thinking that I'd let epoxy harden off for 2 -3 days then grind surface back, clean with acetone and then use fibreglass. Doing this will the fibreglass resin take to the epoxy ok and give me sufficient strength?
 
TamT said:
Im afraid polyester resisns do not adhere very well to epoxy resins. It may be ok if carefully cleaned and abraided.

Colin
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No it wont! You were right in the first place. Epoxy over Polyester is OK. Polyester over Epoxy is a No No
 
Fibreglass over Epoxy

boatmike said:
No it wont! You were right in the first place. Epoxy over Polyester is OK. Polyester over Epoxy is a No No
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Sorry Mike but I beg to differ?

I have personally done so, having had to do a major coach roof repair to the mast step having replaced the actual mast step, I did exactly that. The mast step molding was cut open to expose the core which was 1 1/4" of marine ply moulded between the coach roof and the inner fiber glass molding. Having re laminated the marine ply core using Top grade marine ply and layer upon layer of heavy weight woven mat and Biaxial cloth set in epoxy, this was brought to a near finished level.
Having researched on the net and found out from West Epoxy site I found articles relating to this very question. See link
http://www.westsystem.com/ss/polyester-over-epoxy/
If you read the article you will see that what is most important is, that the epoxy should be fully cured?
There test suggested two weeks minimum?
I personally waited 3weeks with the coach roof, in sunny, warm, and breezy at times summer weather back in 2010 so was very lucky to have good possible curing time?
I know to date, it is only 18mnths, but it is showing no sign of adhesion defects after the freezing winter of 2010 and this summer’s temperatures here in the south west.
West’s conclusion :

The overall effect of moisture on the gelcoat's bond to both the polyester and epoxy panels was negligible. The graphs show very little change in the adhesive strength, and much of the variance is within the tolerance of the test method. When the studs were pulled, the failures occurred in the fairing compound, cohesive failure of the gelcoat, or in the bond between the gelcoat and laminate. Since these three modes of failure occurred under similar loads, it indicates that the strength of the bond is close to the cohesive strength of both the gelcoat and the fairing compound. The nine-week exposure may not simulate what could happen after many years, but it does indicate that the gelcoat bond to epoxy laminates is a good bond and should perform well.
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.
 
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Polyester on Epoxy

As said if the epoxy is fully cured it is no more or less than a lump of GRP. (inert) It could be Aluminium steel or wood. There is no chemical bonding just a bond onto the grain or roughness of the epoxy.
The polyester will not stick very well but if the mechanical design is good then it will be quite OK. I mean if the ployester and cloth a wrapped around the epoxy even a complete disbond will not matter very much.
If you use epoxy of course then if the base layer is still not fully cured you can get a chemical bond whci really sticks. good luck olewill
 
William_H said:
As said if the epoxy is fully cured it is no more or less than a lump of GRP. (inert) It could be Aluminium steel or wood. There is no chemical bonding just a bond onto the grain or roughness of the epoxy.
The polyester will not stick very well but if the mechanical design is good then it will be quite OK. I mean if the ployester and cloth a wrapped around the epoxy even a complete disbond will not matter very much.
If you use epoxy of course then if the base layer is still not fully cured you can get a chemical bond whci really sticks. good luck olewill
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Oh dear. Yet again a simple answer is not sufficient......

To be pedantic you CANNOT achieve a chemical bond putting polyester over epoxy. You CAN achieve a mechanical bond of sorts but it is highly undesirable to rely on it and it can indeed cause more problems than it solves. Over time, a lack of chemical bonding can cause voids to develop into styrene rich moisture traps which is a major cause of delamination and blistering. This problem will be at its worst if the area is constantly wet. In a "dry" area above the waterline where high strength is not required you might get away with it but the general advice is to avoid it wherever possible.
I don't believe it's worth the risk for the sake of saving pennies.
 
As a boat builder we would always use Epoxy on Epoxy with the idea of having a better chemical bond no matter the age, it being a new surface will make it adhere even better - as with everything preparation and design are the key elements.
 
Cookee said:
As a boat builder we would always use Epoxy on Epoxy with the idea of having a better chemical bond no matter the age, it being a new surface will make it adhere even better - as with everything preparation and design are the key elements.
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Yes and as a retired boatbuilder myself that's always been my practice. The only exception I have seen is where so called "osmosis" has resulted in an epoxy repair that for cosmetic reasons has needed to be overlaid with a polyester gel coat. Never liked to see this personally but the West System guys do claim it to be possible with the clear caviat that it is only a mechanical bond. Even then, below the waterline, the overlay of gelcoat is undesirable and unnecessary.
 
I read the OPs query to relate to a floor support - if that's a support to some floor panels in the saloon I can't see why what he proposes wouldn't be at least adequate. If twere me I'd go ahead.
If the location was more critical then I'd use epoxy, as I have on my boat.
 
boatmike said:
Yes and as a retired boatbuilder myself that's always been my practice. The only exception I have seen is where so called "osmosis" has resulted in an epoxy repair that for cosmetic reasons has needed to be overlaid with a polyester gel coat. Never liked to see this personally but the West System guys do claim it to be possible with the clear caviat that it is only a mechanical bond.



I know having spoken to a few local Fibre Glass moulders beforehand, that the general consensus is what you and others have said, but for me the only option would have been to paint the repair, and I didn't want to do that. So having done some research online, found the article by West Epoxy, where they did actually say it was possible.

This lead me to apply several coats of coloured Gelcoat, which allowed me later, to cut it back to original levels, without it standing out as a repair?

I have to say that the overcoating of epoxy with polyester, for me, was exactly that, for cosmetic reasons only, the Gelcoat also had to be coloured to try and match existing colour.

I am happy with the overall result, and hope the gelcoat stands the test of time?
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