[Public-list] barrier coat

[George Dinwiddie]

John Malthouse wrote:
> I am working on my A22 and A30 hulls, starting with the A22. My question 
> after reading George's comment is, why barrier coat would increase the 
> chances of osmosis blisters?

Consider how osmotic blisters form:
  1. Water penetrates the gel coat into the laminates.
  2. In side the laminates, the water dissolves and/or reacts with 
chemical components.  (This is called "hydrolyzing.")
  3. The resulting molecules are too big to migrate back through the gel 
coat.
  4. As more water infiltrates (moving from a region of greater 
concentration to a region of lesser concentration), the pressure within 
the laminates builds up.
  5. Eventually the pressure is more than the laminate can bear, and it 
splits in a small area and bubbles outward, forming a blister.

My theory is that the gel coat that Whitby used was fairly low quality, 
and porous enough that step 3 doesn't happen much.  Certainly hulls that 
are blasted or peeled to remove the gel coat show the effects of 
hydrolization in the underlying resin.  It's less solid, more powdery. 
Fortunately the laminate is very thick, also, and hand-laid-up.  The 
deterioration seems to be very slow beyond the outer layer (mat or 
chopper gun, I don't know) that's just behind the gel coat.  I've never 
heard of an Alberg 30 having catastrophic hull failures, though a few 
random blisters is not uncommon.

Now, consider the situation where the outer skin has been made more 
impervious to water.  It's still not completely impervious (not even 
with epoxy or coal tar).  The pores are much smaller, so the water comes 
in more slowly, but the hydrolyzed resin cannot escape.

Also consider that water penetrates the laminate from both sides if 
there is any water in the bilge.  And that any coating you put on is 
highly unlikely to be perfect, without any blemishes.  The odds of 
keeping the hull dry are stacked against you.

Empirically, I have known of a couple boats that had worse blisters 
after a bottom job than before.

Disclaimer: I am not a chemist, though my father was.

  - George

-- 
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   When I remember bygone days                         George Dinwiddie
   I think how evening follows morn;            gdinwiddie at alberg30.org
   So many I loved were not yet dead,           http://www.Alberg30.org
   So many I love were not yet born.
                'The Middle' by Ogden Nash
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