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Interlux Gelcoat Blister Repair FAQ

Question: What do I do if blisters do occur?


1. Properly prepare the Gelcoat - This includes getting all of the antifouling paint off and removing 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 Epoxy Resin - This solvent less epoxy seals up the laminate and fills any cloth that has been voided of resin. It provides a water barrier to minimize the possibility of reoccurrence of damage.

4. Application of InterProtect - This will act as a tie coat to the antifouling.

Question: How do you fix blisters on a fiberglass hull?

Answer: Following Interlux InterProtect 900 Bulletin, assuring blisters have been ground out and the hull is dried, products such as Epiglass can be used as epoxy resins to seal the missing areas of the gelcoat. Then the products Watertite, fairing and filling compounds, may be used if there are any minor areas of gelcoat missing or if there are any surface imperfections present. Once the boat has a dry, smooth, sealed surface and has been adequately cleaned, Interlux Interprotect 2000E barrier coat may be applied, to prevent the occurrence of osmotic blistering problems from in the future.

Question: How do I know if the blistering on my boat is caused by osmosis?

Answer: There is no easy answer to this question but a good Indication is if there are blisters evident in the gelcoat which, when opened, contain a brown liquid smelling of vinegar. We would, however, recommend that you have a surveyor look at the boat.

Question: I've just bought a nearly-new fiberglass boat. What's the best treatment to protect against blisters in the future?

Answer: Protection is always better than cure and it really does make sense to protect a new boat as well as an older craft. We strongly recommend that you use the Interprotect system. This is a multi-coat system of an extremely waterproof epoxy primer specially formulated for this purpose.

Question: What is blistering?

Answer: The diffusion of fluids through membranes or porous partitions. It's a boat owner's greatest enemy. Water absorbs through the gelcoat causing damage and weight gain.

Question: Why will InterProtect protect my boat from blisters?

Answer: Because InterProtect is based on unique epoxy resins, it has very high water resistance properties and acts to seal the hull from the water. InterProtect is the only Blister Protection and Prevention coating that uses Micro-Plate® Technology. Microplates use a water barrier system similar to shingles on a roof, eliminating water intrusion.

Question: What actually causes the blister?

Answer: As moisture enters the hull it reacts with some of the components forming the laminate. This produces small regions containing a mix of highly concentrated chemicals. This concentration of chemicals is stronger than anything on the outside of the hull i.e. the seawater. This leads that cell to set up a powerful osmosis force that pulls more water into the hull from the outside in an attempt to lower the concentration. This starts to form pressure within the hull as more water is pulled in than can be contained without some expansion taking place. The result is blistering. Very often if you burst a blister the contents will spurt some distance indicating the high pressures involved.

Question: How do I recognize gelcoat blisters?

Answer: Blisters are the most common warning sign. They can vary from small pinhead blisters to areas as large as the palm of a hand. The presence of any fluid behind a blister indicated 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. Identification of osmosis should be followed with immediate professional examination.

Question: Apart from blistering what other warning signs should I look out 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 gelcoat and can cause "wicking" where water is drawn into the hull by capillary action.

Under-curing 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.

Question: How do I protect my boat from getting blisters?

Answer: You should protect your boat with Interrotect before it ever goes in the water. Protection is always better than cure and it really does make sense to protect a new boat as well as an older craft. To achieve this protection it is necessary to sheath the hull with a water barrier to seal the surface. This is done over the existing gelcoat. There is no better time to apply an anti-osmosis system than when the boat has not yet been launched. However, it must be stressed that protective systems cannot stop osmosis once it has started, or prevent it from occurring in poorly constructed hulls. It is important that a full check is undertaken before starting. Remember as well that moisture may get into your hull from the bilges so always ensure good ventilation inside the boat and either keep the bilges dry or paint them well to seal the surface from moisture permeation.

To learn more about Interlux paint visit

Bottom                          Antifouling
     Blister Repair & Prevention

Interlux Antifouling Basics
How Antifouling Paint Works
Interlux Antifoulant Selection Guide
Painting Bare Fiberglass Bottoms
Antifouling Paint Systems for Various Substrates
Handy Specifications
How To Paint Previously Painted Bottoms
Interlux Applying Antifouling Advice
Antifouling Paint Calculator
Antifoulant on Aluminum Hulls
Interlux Antifoulant on Metal Outdrives, Keels and Centerboards
Hints for the Perfect Bottom Finish
Interlux Antifoulant FAQ
What is Osmosis Gelcoat Blistering
How to Recognize and Treat Osmosis Gelcoat Blisters
Protect Against Gelcoat Blisters with Interlux Interprotect
Interlux Gelcoat Blister Repair FAQ

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