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  #21  
Old 06-17-2010
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Jeff -
Im a Physical Chemist, among other things. Ive observed over the years that the manufacturers recommended thickness of applied barrier coats has greatly increased over the past 20-25 years. Vapor permeability is dependent on 'thickness' of the epoxy barrier and those barriers applied 'in the earlier years' of relatively 'thin' recommendations have or are beginning to fail; the 'rate' of vapor permeation dependent on the average temperature of the ambient water. Once the water vapor 'can' penetrate there are 'many' causes for the vapor to 'stay behind' the barrier - chemical, mechanical, etc. but the principal mechanism seems to be 'discontinuous' lay-up where there are bonding discontinuities due to 'curing' between the laminate layers ... the action of simple hydrolysis of the internal structure; hydrolysis being equivalent of 'rusting' of (vapor porous) polymers.
THICK barrier coats better retard the vapor permeation; thus, better protecting vs. the inevitable 'hydrolysis'. In industry, FRG tanks that are laid-up in a single continuous process have much better resistance to hydrolysis damage and therefore last longer and with minimum 'blistering', etc.
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Old 06-17-2010
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Quote:
Originally Posted by RichH View Post
Jeff -
Im a Physical Chemist, among other things. Ive observed over the years that the manufacturers recommended thickness of applied barrier coats has greatly increased over the past 20-25 years. Vapor permeability is dependent on 'thickness' of the epoxy barrier and those barriers applied 'in the earlier years' of relatively 'thin' recommendations have or are beginning to fail; the 'rate' of vapor permeation dependent on the average temperature of the ambient water. Once the water vapor 'can' penetrate there are 'many' causes for the vapor to 'stay behind' the barrier - chemical, mechanical, etc. but the principal mechanism seems to be 'discontinuous' lay-up where there are bonding discontinuities due to 'curing' between the laminate layers ... the action of simple hydrolysis of the internal structure; hydrolysis being equivalent of 'rusting' of (vapor porous) polymers.
THICK barrier coats better retard the vapor permeation; thus, better protecting vs. the inevitable 'hydrolysis'. In industry, FRG tanks that are laid-up in a single continuous process have much better resistance to hydrolysis damage and therefore last longer and with minimum 'blistering', etc.
For my purposes then, thin is better- I need some permeability as the planks will still need to take up.
I may have to deal with a lot of things, but fortunately, blistering is not one of them.

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  #23  
Old 06-17-2010
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I can't see the need for a barrier coat on a wood boat. Anything else, yes. Wood? No...
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Old 06-17-2010
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I can't see the need for a barrier coat on a wood boat. Anything else, yes. Wood? No...
Talk to Bob Derecktor. he smeared it the first time so he must have had some ideas as to why. I can see that it would moderate the rate of transpiration so it's not necessarily a bad thing- why did they C-flex Oh Joy?
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Oh Joy suffered from electrolysis of fasteners only 18 months after being refastened due to a very hot marina. The owners at that time decided to refasten one last time and chose C-Flex over other sheathing methods to protect her from both marine borers and having to refasten again, with all of the issues that would entail. Unfortunately, going with canvas over ply in the traditional fashion for the deck, subsequent substandard maintenance, repairs and after they sold her, a total lack of maintenance, caused her current woes from topside fresh water leaks.
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Quote:
Originally Posted by CharlieCobra View Post
Oh Joy suffered from electrolysis of fasteners only 18 months after being refastened due to a very hot marina. The owners at that time decided to refasten one last time and chose C-Flex over other sheathing methods to protect her from both marine borers and having to refasten again, with all of the issues that would entail. Unfortunately, going with canvas over ply in the traditional fashion for the deck, subsequent substandard maintenance, repairs and after they sold her, a total lack of maintenance, caused her current woes from topside fresh water leaks.
Nuts.
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