Osmosis Treatment & Barrier Coats
Osmosis is the slow disease of fiberglass hulls. Water permeates through the gelcoat over years, reacts with the underlying laminate, and creates blisters. On most modern boats it's manageable; on older boats in warm tropical water it can become serious. Treatment is expensive, time-consuming, and usually best done at a specialist yard.
The treatment process
- Peel: Mechanical removal of gelcoat (gelcoat peeler) or sanding. Goal: expose laminate.
- Wash: Pressure-wash to remove water and contamination from the laminate.
- Dry: Time. 6 weeks to several months. Moisture meter readings track progress. Target ~5%.
- Fair: Fill blisters and damaged areas with epoxy fairing compound.
- Barrier: 5–7 coats of epoxy barrier paint (Interprotect, Hempel Light Primer, etc.).
- Optional re-gelcoat: Cosmetic finish above the waterline; below, not necessary.
- Bottom paint: Standard antifouling on top of barrier.
What it costs
| Service | Common range (USD) | Notes |
|---|---|---|
| Preventive barrier coat (40 ft) | $1,500–3,000 | 3–4 coats during routine haul |
| Light osmosis treatment | $5,000–8,000 | Spot peeling, barrier, paint |
| Full osmosis treatment (40 ft) | $8,000–15,000 | Full peel, dry, barrier, paint |
| Severe osmosis (extensive damage) | $15,000–25,000+ | If extensive laminate damage |
| Mediterranean yard premium | 50–70% of US/UK price | Marmaris, Preveza, similar |
Where to get it done
Cruisers report consistent quality at: Marmaris (Turkey), Cleopatra Marina Preveza (Greece), Lagos (Portugal), Cape Town (South Africa), Whangarei (New Zealand), and the major refit yards in the UK and US. Bottom-up: any yard that does serious fiberglass work can also do osmosis.
Frequently asked questions
What causes osmosis?
Water penetration through gelcoat into laminate; chemical reactions create blisters.
Surface vs serious?
Tiny under-paint blisters often hydration only. Deep gelcoat blisters with vinegar smell = osmosis.
Treatment process?
Peel → wash → dry → fair → barrier → paint. Months long.
Cost?
Full treatment $8,000–15,000 (40 ft). Med/South Africa yards 50–70% of US/UK.
Preventive barrier worth it?
On modern boats, yes — $1,500–3,000 during haul-out.