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Old 14-06-2009, 08:06   #1
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Two Vapour Barriers?

I was working on my boat yesterday and a fellow Sailor stopped by to chat. Describing what I was doing (painting the inside of the hull) he told me a story where a friend of his did this and had trouble with blistering because he had trapped moisture by using the wrong paint (he had used exterior hull paint) and created two vapor barriers (or similar), one at the outside face of the hull and one on the inside face of the hull. He told me that this person actually had to remove ALL the paint from the interior!! I can't help but think that part of this story is missing.
I do know that two vapor barriers can be a problem in buildings but is it applicable with fiberglass boat hulls? I appreciate that in wood hulls this could be a problem but fiberglass boat hulls?
A little background:
  • The intent is to make a very cleanable smooth surface inside the boat
  • It is a vinylester hand layup hull
  • It has an airex core in some/most areas
  • I was going to paint one coat of West Epoxy on the interior to do some smoothing and then two coats of Awlgard after that
  • I'm not laying in any fiberglass in this, just painting
  • The survey of the boat when I bought it had no moisture issues in the hull
Can Anyone tell me if they think this will be a problem?

Much Thanks,
Extemp.
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Old 14-06-2009, 08:26   #2
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Like you I know that two vapor barriers in housing construction can be a big problem, but I really cant see it a problem on your boat, although in theory I suppose its possible to trap that moisture....does that translate into blisters and structural problems.....seems unlikely.

BTW...I've seen a couple pics of your work and it looks like a fine job you're doing.
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Old 14-06-2009, 10:31   #3
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Quote:
Originally Posted by James S View Post
Like you I know that two vapor barriers in housing construction can be a big problem, but I really cant see it a problem on your boat, although in theory I suppose its possible to trap that moisture....does that translate into blisters and structural problems.....seems unlikely.

BTW...I've seen a couple pics of your work and it looks like a fine job you're doing.
As everything in this world equalizes, I'm sure there is some effect. Having said that, there is a difference when talking about absorbing. I would think that if you put a moisture barrier (or multiple) anywhere along the transmission/equalization path, it would stop or help stop the flow. I would also think that if the moisture is making it to the interior face or your hull and it's NOT condensation, your hull must be saturated.
Anyone?

I'm having a great time working on and figuring out all the systems in my boat and ...... trying to do the best job I can.

Thanks,
Extemp.
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Old 14-06-2009, 12:03   #4
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I'm having a great time working on and figuring out all the systems in my boat and ...... trying to do the best job I can.

.
That describs my feelings exactly!!
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Old 15-06-2009, 02:49   #5
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Good question; but I don’t think a properly prepared and applied paint (even barrier coat) will cause any blistering problems.

Quote:
Originally Posted by Extemporaneous View Post
1. As everything in this world equalizes, I'm sure there is some effect ...
... 2. I would also think that if the moisture is making it to the interior face or your hull and it's NOT condensation, your hull must be saturated.
1. Since everything allows some water vapour to diffuse through it to some degree, the newer term "Vapour Diffusion Retarder" is more accurate than the old “Vapour Barrier” terminology. The ability of a material to retard the diffusion of water vapour is measured by units known as "perms" or permeability. A perm, at 73.4̊F (23̊C), is a measure of the number of grains of water vapour passing through a square foot of material per hour at a differential vapour pressure equal to one inch of mercury. Any material with a perm rating of less than 1.0 is considered a vapour retarder.

2. Exactly!

Some application tips to prevent bubbles & blisters:

Avoid painting in direct sunlight or application to hot surfaces. When any paint is applied to a hot surface the solvents vapourize very quickly. At the same time, the paint is drying, forming a film and becoming resistant to the transfer of the solvent underneath. This creates a bubble. Excessive thinning combined with heat will increase the chances of this condition occurring.

Allow the surface to dry completely before priming and painting. Obviously, moisture underneath the paint film can cause a lack of adhesion.

Allow 4 or more hours of drying before predicted adverse weather. Exposure of the paint film to rain, dew or high humidity shortly after application, and during the drying phase, can also cause paint blisters.
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