Practical Boat Owner - Paint and Glassfibre

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Paint and Glassfibre - Pure epoxy

question

Why do you need 100% epoxy for osmosis treatment?

G.S.Bougourd

answer

From your question, I assume that you mean why is it necessary to use 100% solids (or solvent free) epoxy when carrying out osmosis treatments? Very simply, solvent free coatings are used to avoid the risk of organic solvents being absorbed (and trapped) within the laminate, which is usually very absorbent at this stage. Solvents are always undesirable in laminates as they can weaken the laminating resin, and may damage the protective epoxy coating scheme as well as encouraging moisture retention. However, once the initial, solvent free coating has been applied to the bare laminate, it can usually be followed by either solvent free or solvent containing materials, as there is no longer any risk of solvent being absorbed. So if (for example) you were using International's Gelshield system, once the initial coat of Gelshield solvent free is applied, it can be followed either by further similar coats, or by a solvent containing epoxy such as Gelshield 200 or Interprotect. Likewise, solvent containing coatings can be used for osmosis prevention (i.e. where the gelcoat is still present), as the laminate is not exposed. Other types of coating (such as polyurethanes) can also be used for osmosis treatment and prevention, although their moisture barrier properties are not usually as good as a well formulated epoxy.