Epoxy vs Osmosis ??

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Hello
Can a GRP boat that has been treated with Epoxy suffer from Osmosis for tha sake of endless ifs and buts lets consider an older boat that has suffered in the past but has under gone a professional peel re-sheath and epoxy. Better than original ?
 
Better than original, but can still suffer with osmosis, thats why they dont give them a long gaurantee.
 
All the available resins are to some degree permeable to moisture and a poor quality moulding that contains voids and pockets of styrene will over time absorb moisture. The less absorbent the surface layer is, the longer the process will take. The common materials are, in order of permeability:

Orthophthalic polyester (worst, never used below water today)
Isophthalic polyester
Vinylester (polyester)
Epoxy (best)

Naturally that is also the order for cost!

So yes, an epoxied hull can get osmosis but it will take many times longer than before so the boat will probably be scrap before it happens.
 
Is the type of polyester stated on the can, do you know? I have not noticed anything in the catalogues that clarifies the type of polyester and don't ever recall seeing vinylester listed as such.
 
I have always wondered about epoxy coating the hull - I know nothing about it, but it seems to me the fact that it is less porous also means that any moisture that does get in is more effectively sealed in, and it would take the hull longer to dry out in winter. Maybe it's better that the hull "breathes" a bit, provided it's given a chance to dry out over winter?

The above is a question, not informed opinion!
 
Often the type is given by codes rather than words. You need to ask the suppliers for a particular type. I would hope that the small quantities of resin sold in chandleries is isophthalic but I wouldn't like to bet on it.

More specialist resins need to be obtained from the big suppliers eg: here
 
G'day Simon,

Re: <<I have always wondered about epoxy coating the hull>>
And <<it seems the fact that it is less porous also means that any moisture that does get in is more effectively sealed in,>>

You need to consider the following:
Older polyester resin boats were made with chopped strand mat (CSM) and much of this was applied with a chopper gun, as the name implies the gun chopped the glass and sprayed it onto the mold adding polyester resin and catalyst in one operation. There are some problems with this system, depth, mixture, ratios and so on, but the bottom line is that fibreglass in a hollow straw and using lots of short strands with open ends to transport moisture into the laminate is a disaster waiting to happen.

Add to this the fact that the moisture enters under pressure (below the water line) and has to dry naturally at the coldest and wettest part of your year, not looking good is it.

So "provided the hull is dry" when treated it should last many years as Snowleopard has pointed out.
Treatment should include at least one layer of rovings to provide a cover for the newly exposed ends of the old fibreglass, and at least 4 coats of epoxy resin.

Hope this helps

Avagoodweekend.
 
Yes it can ~ quite often a new coating over an old hull will bring on symptoms of osmosis that were not apparent previously.
Epoxy is good stuff, but however good it is it cannot help prevent osmosis if it has been put on over a damp hull, or a hull that still has the products of hydolysis trapped within it.
Unless the hull is "properly prepared" leave it alone.
 
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