Sea-Line Antiosmotic HS Epoxy Primer 3:2 – High-Build Epoxy Osmosis Barrier

Sea-Line Antiosmotic HS Epoxy Primer 3:2 is a special high-solids primer designed to combat osmosis in fiberglass (GRP) hulls. Osmosis is the process of water penetrating the gelcoat and forming blisters within the laminate, which can compromise the boat’s structure. This primer is engineered to halt that process – it seals the porous fiberglass and restores the surface after osmotic damage has been removed. Sea-Line 3:2 HS Primer is solvent-free, meaning that during curing it does not release any thinners that could remain in the laminate. The result is a dense, fully epoxy layer strongly bonded to the substrate and completely waterproof.

• Application – Osmosis Repair: In an osmosis repair, first the affected hull areas are identified (blisters opened up), the damaged laminate is removed, and the area is neutralized/dried. After prep, Antiosmotic HS Primer 3:2 is applied as the rebuilding primer on the clean, dried-out laminate. Due to its thixotropic (non-sag) consistency, the primer allows thick application up to 150 µm dry film per coat, which is crucial for filling in the roughness and pores after removing the osmotic material. Two coats of the product (approximately 300 µm total DFT) provide a reliable barrier that rebuilds the protective thickness and guards against the return of osmosis. Apart from active osmosis repairs, this primer can also be used proactively – on new boats or during preventative gelcoat maintenance, to add an extra epoxy barrier layer under the antifouling.
• Solvent-Free Formula – Advantages: Being solvent-free means the primer has 100% solids by volume, giving you maximum film build per coat (no evaporation reducing the film). It also eliminates the risk of any residual solvent being “trapped” in the laminate – important for osmosis since such solvents could potentially cause blisters down the line. The cured layer is very hard and strong, yet sufficiently elastic (thanks to the HS formulation) to accommodate hull flex without cracking. The primer is dark grey in color, providing good contrast when applied over a light gelcoat or previous white primer, so it’s easy to see where it’s been applied. Thanks to its excellent self-leveling properties (even in thick layers, it flows out nicely), the finished primed surface is smooth and ready for additional priming/painting with minimal touch-up.
• Application & Systems: Sea-Line Antiosmotic Primer 3:2 is mixed at a 3:2 volume ratio (or 100:67 by weight) – the kit contains pre-measured quantities of Component A (base) and Component B (hardener). Stir the mixture thoroughly; it is viscous but still can be applied by brush, roller (for heavy coats a short-nap roller is recommended), or airless spray (conventional spray is generally not suitable due to the high viscosity). For brush/roller, you may add up to 5% epoxy thinner to improve flow, but avoid adding more (the product is designed for maximum build without thinner). The working time of the mixed batch is around 45 minutes at 20°C – keep this in mind and mix smaller quantities if working in warm conditions or at a slower pace. Apply the first coat carefully, working it into the pores and roughness of the laminate. Allow it to cure to touch (~8 hours at 20°C for this HS primer). Apply a second coat to reach the recommended total film build. No sanding is necessary between coats as long as you apply the next layer within 8 hours – 3 days (at 20°C). If you exceed that window, a light sanding to scuff the surface is needed before continuing. After the final coat, let the primer fully cure (minimum 7 days at 20°C) before heavy service or before launching the boat, or follow it with an additional epoxy or polyurethane primer/paint as directed by your coating system.
• Integrating into a System: For complete osmosis protection, Antiosmotic HS Primer 3:2 is typically applied after the hull has been primed with a thinner epoxy primer like Sea-Line Lightprimer 5:1 or Epoxy Primer 4:1 on the bare laminate – the thin primer penetrates and “seals,” and the high-build HS primer on top of it provides the necessary thickness for a barrier. After applying the HS primer, you can add another coat of Lightprimer 5:1 (which is easier to sand) to perfectly fair the surface before paint or antifouling. Finally, the antifouling paint is applied. This entire system, finished with Sea-Line Antiosmotic HS Primer, ensures maximum protection of the fiberglass boat against water ingress and osmotic damage. By using this product correctly, your osmosis repair will be long-lasting and effective, restoring the hull’s integrity.

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