Blistering caused by hydrostatic pressure

What?

“Blistering” is the formation of “Bubbles” in the exterior decorative paint film, resulting from localised loss of adhesion and subsequent lifting of the existing paint film from the underlying surface. This fault is not attributable to the paint itself but rather to the substrate conditions.

“Hydrostatic Pressure” is the pressure exerted by water onto a surface from a specific height which increases in proportion to depth measured from the ground surface because of the increasing weight of fluid exerting downward force from above due to gravity.

Hydrostatic Pressure can be caused by numerous conditions including high water table, excessive storm water run-off flowing towards a wall or simply poor drainage systems. As the pressure increases so does the risk of blistering of the painted surface due to moisture entrapment behind the paint film. 

Why?

As moisture travels through a porous masonry substrate that is sealed at the exposed surface by paint, the moisture simply cannot escape quickly enough from beneath the paint film, resulting in the formation of Blisters.

The blisters can appear worse or more intense as the pressure increases. In addition, as the moisture converts from a liquid into a vapour when the surface is heated by the sun, the moisture vapour expands but cannot escape through the paint film hence blisters will increase in size or intensity.

How does it occur?

When the painted surface is subjected to excessive amounts of moisture it causes the paint film to swell it occur and eventually lose its adhesion to the substrate. This may appear worse during wet weather periods as the blisters fill with water. Blistering of the paint film due to swelling is most likely to occur within the first few days in the life of the completed project.

The coating systems ability to breathe and to allow moisture to pass slowly through the dry paint film is a characteristic called “Water Vapour Transmission”. However, even the most effective coatings formulated to the highest quality can only withstand a certain amount of hydrostatic pressure before blisters start to form. In simple terms; the coating can’t breathe fast enough to let the moisture out quickly.

A typical example of the damaging effects of hydrostatic pressure on a paint coating can be observed on a masonry retaining wall which has NOT had an effective waterproofing system applied.

The moisture from the soil and garden beds behind the retaining wall will pass through the wall and attempt to escape through the paint coating on the exposed surface.

If the hydrostatic pressure is too high then the moisture will be unable to escape effectively or quickly enough resulting in the formation of blisters.

Other typical examples of where increased levels of hydrostatic pressure cause problems include...

• Planter Boxes

• Retaining walls

• Basement areas

• Underground Car Parks

• Building Eaves (from overflowing or blocked gutters)

• All areas where soil or earth is backed against a porous structure

Solution

The correct way to ensure that the problem is rectified is to take ALL the necessary steps possible to contain, divert or eliminate the moisture at the source of entry to the substrate.

All areas where the soil or earth is backed against the structure, such as a retaining wall, would require the soil to be removed completely so that the rear of the wall can be fully waterproofed and other suitable measures, such as “weep” holes and air-gaps, can be installed to allow the moisture to escape effectively.

In order to FIX the problem once it occurs, the extent & causes of the problem need to be clearly identified, the causes removed and the old paint stripped back to a sound substrate before any new painting commences. Failure to do this will only result in further blistering!

The presence of blistering is a good indication that the adhesion of the paint film has now failed therefore simply repairing the blisters (only) is unlikely to cure the problem. Once blistering occurs, it is quite likely that more blisters will appear in future unless the true cause of the moisture ingress is identified and eliminated.

Prevention

Structures to be painted should be always be sealed or insulated from moisture ingress by the inclusion of air gaps, by the use of moisture barriers or by the use of effective waterproofing.

Waterproofing systems need to be employed on all bare surfaces in contact with water or in-ground moisture to prevent seepage and the passage of water under hydrostatic pressure. Waterproof filmforming coatings are formulated for above and below ground, as well as for interior and exterior applications.

A water-repellent coating is ineffective at preventing the passage of water under hydrostatic pressure. These exterior coating systems are mainly designed for above ground concrete or masonry to temporarily repel water. Some water-repellents are film-forming and others simply act by filling surface pores to prevent moisture movement. They should NOT be used to prevent water ingress due to hydrostatic pressure.

Moisture from the atmosphere can also be present in the substrate prior to painting but this is not necessarily due to hydrostatic pressure. Regardless of the origin, if moisture becomes trapped beneath the paint film it is likely to force its way out when the sun heats the surface, resulting in blistering due to adhesion loss. So the substrate moisture levels always need to be checked before painting! 

If the surface to be painted appears damp, locate and remove the source of moisture where possible or water-proof the surface where the moisture enters the substrate.

Golden Rule: If the moisture content of masonry is above 15%, it is unsuitable for painting and must be allowed to dry- out!

A portable “Moisture Meter” can quickly determine what the current moisture levels are. Details on the correct use of this instrument are provided in a separate bulletin. After painting, the moisture levels within the substrate need to remain low to avoid blistering. If the levels rise again due to water ingress, seepage, rising damp or damage to the paint film, then blistering is likely to occur.

References

A brief explanation of the “Waterproofing” requirements for buildings prior to painting can be found in Australian Standard AS/NZS 2311 Section 2.1.2 “Guide to the painting of buildings” 

For information on products or systems suitable for waterproofing contact either Dulux® AcraTex® (www.acratex.com.au or http://www.dulux.co.nz/applicator/our-brands/dulux-acratex) or Parchem (www.parchem.com.au or www.parchem.co.nz) for a specification or advice.

Detailed information on the preparation of unpainted masonry and concrete surfaces prior to painting can be located in Australian Standard AS/NZS 2311 Section 3.9.1 and 3.9.2 “Guide to the painting of buildings”

Information on “Water Repellents for Concrete and Masonry” can be found in AS/NZS 2311 Section 4.17 “Guide to the painting of buildings”