What is fiberglass rot?

Fiberglass rot, or "osmosis," refers to a phenomenon where, when a semi-permeable membrane exists between two liquids of different concentrations, one of the liquids penetrates the membrane and causes an increase in pressure within the membrane.

How does "osmosis" affect fiberglass boats?

In the case of a boat, the semi-permeable membrane is the gelcoat, and the liquid is water. Over time, water is absorbed into the fiberglass laminate. Although most of the moisture passes through the hull into the bilge without causing significant damage, some of it begins to break down the materials used in the laminate, leading to the formation of "osmotic" solutions. Osmotic solutions contain acetic acid and glycol, and their molecular weight is greater than water, which prevents them from passing back through the gelcoat or laminate. As more water is absorbed into the fiberglass, osmotic pressure builds up, eventually causing blisters to form in the gelcoat. Most unprotected fiberglass boats will show signs of osmosis at some point in their lifespan. The timeline and extent of their formation depend on several factors, such as temperature and water type, the length of dry storage periods, and, above all, the quality of the original laminate.

How do I identify fiberglass rot?

The formation of blisters in the gelcoat is one of the first signs of fiberglass rot. When such an osmotic blister is punctured, a slightly vinegar-smelling liquid will drain from it. It is possible to identify fiberglass rot before visible signs appear by checking the moisture content of the hull with a moisture meter. However, this only provides an indication of the problem. If moisture is detected, it is advisable to ask a professional in the field to determine the extent of the problem and suggest corrective actions. Experts have moisture meters that can measure the moisture in the hull. Below are a couple of common moisture meters and their readings when the laminate is dry enough to be painted with Hempel High Protect. Tramex Skipper – use scale 2 – the reading should be 15 or less. Sovereign – use scale A – the reading should be 5 or less.

Protection against fiberglass rot, prevention of fiberglass rot

New boats:

The best way to prevent fiberglass rot is to use the correct methods and materials during boat construction.
The higher the quality of the fiberglass laminate, the better it is protected from fiberglass rot.
Hempel High Protect provides extremely effective protection for a new hull.

Old boats:

Before an old boat can be protected with an epoxy coating, the condition of both the laminate and the gelcoat must be determined.
If you are unsure of their condition, ask a professional for advice.
If the hull is in good condition, it can be protected from fiberglass rot with Hempel High Protect.
Otherwise, it is recommended to repair it properly.

Protection against fiberglass rot and necessary pretreatment

New boats:

The new gelcoat must be thoroughly cleaned with Hempel Degreaser or Hempel PreClean to remove mold release agent residues.
Further information can be found under Surface preparation: cleaning and degreasing. After degreasing, the surface is sanded with sandpaper of grit 60–80, or with light wet sandblasting. Then the surface is rinsed with fresh water. After drying, the surface should be uniformly matte.

Old boats:

Remove all old paint or antifouling paint by scraping or sandblasting/wet sandblasting (this is a quick way to remove old paint layers and at the same time create a rough surface that only needs light pretreatment).
Regardless of the method used, the hull surface should be uniformly matte after treatment and free of traces of old coatings.
Rinse the hull with fresh water and allow to dry.
Fill small cracks and marks with Hempel Epoxy Filler.

NOTE! Proper surface preparation is a prerequisite for a durable and protective surface. So, don't rush this step.

Fiberglass rot preventative paint system – Option 1

Fiberglass rot preventative paint system – Option 2

Repair of fiberglass rot

When is repair of fiberglass rot necessary?

If the laminate and/or gelcoat is in poor condition and shows signs of fiberglass rot, it may need thorough repair.
Complete treatment involves removing the gelcoat, which, under the right conditions, allows the hull to dry sufficiently.
The bare laminate is then coated with Hempel High Protect, which replaces the removed gelcoat and provides the hull with a moisture-resistant surface.

Repair of fiberglass rot and necessary pretreatment

The gelcoat must be completely removed to allow the hull to dry.

Use of power tools:

A grinder is an inexpensive tool for removing gelcoat and pretreating the surface. The disadvantages are slow work and a large amount of dust. The user must wear protective equipment, especially a respirator and safety glasses.

Sandblasting or wet sandblasting:

Gelcoat is easy to remove with sandblasting or wet sandblasting, and at the same time, the hull can be pretreated for the application of Hempel High Protect.
Blasting reveals all defects in the hull, i.e., damage to the laminate.
Pretreatment with these methods should be left to a professional with proper equipment.

Gelcoat removal:

Gelcoat is easy to remove with a specialized device that removes the old gelcoat to a predefined depth and leaves the surface smooth and even.
This is the most effective and best method for removing an old gelcoat surface.
Once the gelcoat has been removed, the surface is sanded with a sanding disc (grit 40) or by using light wet sandblasting to create a sufficiently rough substrate for the application of Hempel High Protect.
Using this method requires professional expertise.
Once the gelcoat has been removed, the entire hull must be steam cleaned or at least cleaned with a pressure washer to remove salts and other impurities that have formed in the laminate due to fiberglass rot.
Salts and other impurities continuously rise to the surface, which is why the hull is washed several times throughout the day.
On average, it takes four weeks to three months for the hull to dry sufficiently before it is ready for coating.
Expert advice at this stage is to ensure that the hull has been sufficiently prepared for the application of Hempel High Protect.
All minor cracks and dents in the hull must be filled with Hempel Epoxy Filler after the first coat of Hempel High Protect.

Fiberglass rot repair painting system

Note! If the recoating time is exceeded, the surface must be sanded and rinsed with fresh water. Proper surface preparation is a prerequisite for a durable and protective surface. So, don't rush this step.

Also read the article on using Hempel High Protect!