All vessel types and construction methods have advantages and disadvantages. Corrosion of many types can appear over time. Luckily modern techniques can prevent and correct the effects of corrosion types. Of course, additional costs are associated with this, but that is the joy of owning a boat!
Wooden boats can develop rot or electrolytic problems. Steel can develop rust, and of course osmosis can develop in fibreglass craft. Although found more often in older craft, osmosis is treatable if not to severe. The cost of these repairs can effect the purchase price of a vessel, but must be taken into account for obvious reasons.
Many theories exist about osmosis, but simply can be described as the penetration of moisture through the substrate into the laminate. Moisture can penetrate the substrate through poor manufacturing systems and products, poor working conditions, and poor environmental conditions. Once the moisture is present in the fibreglass laminate, it will chemically react with the resins in the laminate. Cosmetically this can be seen on the exterior by the presence of small, blister-like abnormalities. If penetrated with a sharp chisel or similar tool, an acrid chemical smell will be detected. A surveyor can also use a moisture meter to detect and confirm osmosis being present.
Osmosis repairs are often carried out in haphazard methods due to a lack of knowledge, a lack of finance, and a hurry to sell the vessel. Often I will survey a vessel where each individual osmosis bubble has been ground out and repaired by simply filling with an epoxy or polyester filler. Over time, moisture will again enter these areas and bubbles often appear next to the original repairs! A properly repaired vessel should look unmolested and provide a hull that is structurally strengthened for the suitable requirements.
The common repair method in modern times is to have the affected layers of laminate removed with a specialized electric planer. The hull will then be placed in a clean, humidity controlled environment, washed clean, and allowed to dry to the right moisture content. This process is where the costs can soar as waiting for the correct moisture content can eventually take many months of time!
Once the substrate is clean and to the right moisture content, boatbuilders will then re-laminate the hull with an equal to or more, fibreglass laminate. Commonly vinyl-ester resins are used due to adhesion qualities, strength and lower costs. Correct primers and coatings will be applied prior to anti-fouling.
New Zealand has the advantage of many skilled boat building businesses that can carry out these operations. They should be able to provide an accurate quote for this work. If not, then the purchaser should look at an alternative firm to use.
Osmosis can be extremely extensive or minor depending on the vessel. Like all vessel faults, most osmosis can be repaired and the vessel can still have a long operating life. Like rot or rust, osmosis can and has affected many vessel sales, but the purchaser is recommended to research repair costs. This can then be figured in to the negotiating price of the vessel.