Yacht osmosis and hull blistering: a surveyor's field guide
How to identify, categorise and report on osmotic blistering in GRP yacht hulls — moisture readings, severity grading, and what to recommend to buyers.
Osmotic blistering is the finding buyers fear most and surveyors most often have to qualify. Get the categorisation right and you protect both the buyer and yourself. Get it wrong and you either kill a deal that should have closed, or sign off on a hull that needs £15,000 of treatment.
Here's how to inspect, grade and report on osmosis in GRP yacht hulls.
What osmosis actually is
Water permeates the gelcoat and reacts with uncured polyester resin and water-soluble compounds in the laminate. The reaction produces an acidic fluid that builds up pressure beneath the gelcoat, eventually forming a blister. Severe cases hydrolyse the laminate itself.
Three things drive it: gelcoat quality, time afloat without drying out, and water temperature. Mediterranean yachts that never come out tend to show it earlier than UK yachts that winter ashore.
Inspection sequence
Do this in order, every time:
1. Out of the water, within 4 hours of lifting — moisture readings are most meaningful while the hull is still wet. Readings taken after a week of drying are nearly useless. 2. Walk the hull at eye level with raking light from a torch — blisters as small as 2mm cast visible shadows under raking light that direct overhead light misses. 3. Moisture meter readings in a consistent grid — typically every 1m along the waterline, midline and turn of bilge. 4. Tap test suspect areas — osmotic blisters sound different from cosmetic gelcoat blisters. 5. Lance one or two representative blisters (with the owner's consent) to confirm fluid type. Vinegar smell = classic osmosis.
See our moisture meter readings guide for what readings actually mean.
A simple severity grading scheme
Use four grades in your report — buyers, brokers and insurers all understand them:
- Grade 1 — Cosmetic — isolated surface blisters under 5mm, no laminate involvement, dry
moisture readings. Annual monitoring; no immediate action.
- Grade 2 — Localised — scattered blisters 5–15mm, slightly elevated moisture readings in
affected areas only. Spot treatment recommended within 12 months.
- Grade 3 — Widespread — extensive blistering, elevated moisture readings across multiple
areas. Full gelcoat peel and recoat recommended; budget £8,000–£18,000 for a 30ft yacht.
- Grade 4 — Structural — large blisters, hydrolysed laminate (soft spots when tapped),
saturated moisture readings throughout. Major laminate repair or write-off.
How to phrase findings
Defensible language pattern:
> "Inspection of the hull below the waterline revealed [N] blisters, predominantly in the [area], > ranging from [X]mm to [Y]mm in diameter. Moisture readings taken within [time] of lifting ranged > from [X]% to [Y]% on a [meter model]. The findings are consistent with [Grade N] osmotic > blistering. Recommended action: [treatment]."
Avoid "the hull has osmosis" as a bald statement — it's both imprecise and inflammatory in negotiations.
What to recommend
- Grade 1 — note in report, no action
- Grade 2 — spot grind, fill with epoxy filler, recoat
- Grade 3 — gelcoat peel, dry-out (3–6 months), epoxy barrier coat system
- Grade 4 — specialist consultation; potential write-off
Always note that osmosis treatment is a wet-trade specialist's domain, and the surveyor's role is to identify and recommend — not to specify the repair scope in detail.
Photo evidence
Photograph every grouping of blisters with a ruler or coin for scale. Photograph your moisture meter on the hull with the reading visible. These photos are what your PI insurer will reach for if a claim comes in.
Modern survey software like evalo lets you attach photos directly to each observation during capture, so the "blister photo + meter reading + location note" stays bundled together through to the final PDF. Loose photos in an appendix are half as useful when a dispute lands.