Solve common problems

Blistering or Bubbling

What is it and how do I solve it?

Dome shaped blisters in the dry paint film. Blisters may contain liquid, vapour, gas or crystals. It can easily be avoided using the points below helping to maintain the finish of your boat.

How to recognise it

Rounded blisters of various sizes usually no larger than about 3-4 mm diameter. In a few instances they may be quite a lot larger.

Cause - Prevention - Treatment

Cause 1: Applying paint too thick, too quickly or in hot or windy conditions or a combination of any of them .

Prevention: Reduce amount of paint being applied, add retarder thinner to slow drying rate down

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 2: Air becomes trapped in porous substrates, for example, when applying paint/varnish on timber or recoating aged antifouling paint.
Prevention: Apply product when air temperatures are cooling down.

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 3: Solvent becomes trapped due to over coating too quickly.      

Prevention: Follow label instructions carefully.

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 4: Contamination of substrate with soluble salts before paint is applied.

Prevention:  Ensure that surface is clean and dry prior to application of the coating

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 5: Applied filler or fairing material has air voids close to the surface which can expand and contract with temperature. This leads to positive blisters and then crater type dimples.

Prevention: Apply filler in thinner layers so as not to trap air near the surface.

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings. Feather the edges of any voids in the filler and apply new filler. Allow to cure and sand as required. It may take one or more applications of filler to fill all voids so that after sanding they are all level with the surrounding area. Once all filling work has been completed fresh water wash to remove all contamination. Allow to dry before patch priming and applying finish coatings.


Cause 6: Exposure to moisture during curing.        

Prevention: Keep product dry when curing. Avoid painting early in the morning or late at night.

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 7: Finished coatings above the waterline are subjected to immersion in water such as under seat cushions on cockpit floors and other places where water can pond.

Prevention: Better housekeeping. Stand seat cushions up or place them inside.

Treatment: Scrape open blisters (or entire coating system where required) and sand back to a good firm edge feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required. And ensure water cannot be trapped or pond anywhere


Cause 8: Blistering on steel hulls may occur when you use too many anodes, leading to over-protection.

Prevention: Have the anodes checked by experts.

Treatment: Seek expert advice before doing anything. If you go ahead, repair blisters by scraping open blisters (or entire coating system where required) and sanding back to a good firm edge, feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 9: On the underwater hulls of fibreglass boats it could indicate the onset of osmosis.

Prevention: Have hull checked by experts.

Treatment: Seek expert advice before doing anything. If you go ahead, repair blisters by scraping open blisters (or entire coating system where required) and sanding back to a good firm edge, feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


Cause 10: Blistering on the underwater surfaces of steel or aluminium hulls, could be due to improper preparation of welding seams. Or not removing corrosion products from the surface.

Prevention: Have the hull checked by experts.

Treatment: Seek expert advice before doing anything. If you go ahead, repair blisters by scraping open blisters (or entire coating system where required) and sanding back to a good firm edge, feathering the edges of the intact coatings to give a smooth overlap. Fresh water wash to remove all contamination. Patch, prime and apply finish coatings as required.


More about treatments 

In all cases it’s a good idea to seek expert advice as the blistering problem may not be localised and if so, probably won’t be a simple fix.

●      Ensure that antifoulings are always wet sanded for health and safety reasons. In some instances it might be easier to repair blisters and totally repaint your vessel.


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