How to Recognize and Treat Osmosis and Protect Against Blistering

Before starting a gelcoat prevention or repair system, refer to the Interprotect 900F System Guide. CallI-800-468-7589 to obtain a copy.

How to recognize and treat osmosis

The main symptom- blisters. Blisters are the most common warning sign and if identified, should be followed up with immediate professional examination. Blisters can vary from small pinhead blisters, to areas as large as the palm of a hand. The presence of any fluid behind a blister indicates a potential problem. If the fluid has a pungent, vinegary odor or feels greasy or sticky when rubbed between the thumb and forefinger, there is a high probability of osmosis. Before any treatment is carried out, you need to establish what has caused the problem. We recommend that you seek the advice of a professional surveyor.

Some blisters occur for reasons other than osmosis. They are often evident as a rash of small pinhead blisters or swellings, either locally (often around the waterline) or over the entire underwater area. These blisters are hard and difficult to break and when broken open will be dry, with no odor evident. The likely cause is air voids. This is not a serious problem, but hull moisture levels should be checked before commencement of any remedial treatment.

Other warning signs to look for

  • Star crazing- This effect can occur where the gelcoat is brittle. Fine cracks usually form due to severe flexing or impact damage, allowing water to seep into the laminate.
  • Pinholes- Tiny bubbles present in the gelcoat reduce its effectiveness and promote rapid water absorption.
  • Prominent fibers- Seen protruding beneath or through the glecoat and can cause wicking where water is drawn into the hull by capillary action
  • Undercuring of the gelcoat- Incorrect mixing or application in unsuitable conditions can cause failure to cure properly. This results in porosity and may lead to water ingress


Important steps in the repair of gelcoat blisters

  1. Proper preparation of the gelcoat
    This includes getting all of the antifouling paint off and removal of as much gelcoat as necessary to get the hull dry (i.e. the entire gelcoat or just small areas). A professional, who has looked at your boat, should make this determination.
  2. Drying of the hull
    This is the most critical step in the process. If you do not get the hull dry it will re-blister. We recommend a comprehensive washing and drying procedure.
  3. Application of Epiglass HT9000 resin
    This solventless epoxy seals up the laminate and fills any cloth that has been voided of resin. This should be done before using fillers to fill the voids.
  4. Application of Interprotect 2000E or Interprotect 3000
    These epoxies are used as a water barrier to minimize the possibility of reoccurrence of damage.

The information provided here has been reproduced courtesy of Interlux. Wholesale Marine reproduced this information 08/14/2009.