Blistering or bubbling are often referred or cross referenced as the same thing when actually there can be different causes. When blistering is an osmotic type of fault the term blistering is most commonly used. Non-osmotic causes are most commonly termed as bubbling but no doubt the confusion will always be there making it hard to determine the cause of blistering or bubbles in the finished surface coating.

Osmotic Blistering

Osmosis is the process by which moistures are transferred through a semi permeable membrane in our case the paint coating.

The most common causes of osmotic blistering are the following.

1 Contamination of the steels surface by water soluble salts.

2 Water soluble solvents trapped within the paint film.

3 A more unlikely cause would be thermal gradients, but not so likely in our scenario (restoring cars)


How to avoid Blistering and bubbles

Remove salts from the surface, always assume that the surface is salt contaminated. Salt can come from so many areas, the atmosphere, contaminated blast media, rain etc. A good wash with a water based detergent solution such as Rustbuster SP10 pre paint prep followed by a further wash with Rustbuster Chlor-X will insure that you have done the best you can. For professional and industrial use you can measure the salt contamination before and after cleaning with a Brestle test kit.

Did you know?

A single salt crystal left on the steels surface can excerpt a pressure of 15,000 psi yes that’s fifteen thousand pounds per square inch of pumping power sucking at the paint film to draw moisture into the salt crystal. This will eventually succeed; the wet salt crystal will be the perfect rust cell. Result a blister on the surface of your finished paint. On a nice sunny day this will pop revealing a wet rusty pit, if you are lucky it will be only one spot but more likely to be several blisters on the paints surface. How the heck did that get there? Well now you know, never underestimate the power of a salt crystal to create rust cells.

Avoid apply in extremely hot temperatures, applying too thick a coating in hot temperatures causes the coating to skin and trap solvents. Trapped solvents heat and produce vapour pressure which in turn can cause a bubble to form on the surface. Always follow the maximum wet paint film recommendations in the data sheet. We sell paint thickness measuring combs for only 20p. Particularly recommended when applying a high build coating

Avoid high humidity during application, because drying and curing is delayed as a result. If a second coat is applied too soon solvent can be trapped within the first coat laying dormant and may pop up at a later date when the coating warms on a summers day.

MCU Bubbling in Petrol tank sealants

Excess moisture on the surface can cause a rapid cure reaction within the MCU coating producing carbon dioxide bubbles within the coating, ruining the job.

Two pack Poly bubbling Aliphatic Polyurethanes

Water reacts with the isocyanates in the hardener component of the 2K coating, causing Co2 bubbles from the gassing process. Bubbles in 2K can be minute or foam like.

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