Blistering (Osmosis) of paint coatings applied to exterior wall surfaces/Glass fibre reinforced plastic .
This this problem can be found on both interior and exterior wall surfaces is associated with liquid water or water/moister water coming in contact contact within the building fabric.
Internally it can due to breakdown within the plumbing system. Externally by a fault within the build construction such as poor damp proofing, or water from internal pipework wetting into the exterior building fabric.
Byway of capillary action water from wet or damp areas can move through a semi-permeable substrate. The hydro moisture pressure builds, forming fluid-filled areas (water) within the underlying the porous substrate, such as concrete, bricks, render. These wet areas can also cause the underlying surface i.e. sand : cement mortar to become weak and crumble. This Hydro pressure causes blisters to appear on the reversed side of the structure .i.e. from the inside to outside.
Exterior paint finishings although protecting the painted substrate from the ingress of moisture from external water i.e. rain /washing down they can also be permeable to moisture vapour. Consequently surface blistering occurs and can be due to the underlying weakness of the substrate,
When problems of this nature are experienced, inspection should be carried out to: 1: Remove the surface of the affected area to expose the underlying surface. 2: examine the reverse sire to the sample removed. 3: check with an approved moisture meter. The moisture content of the area removed in relation the overall general area to establish. The condition of the underling surface is also check for density.
This can occur with any coating where moisture is trapped behind it, but it is a common issue with older properties lacking a functional DPC’s
At the same time, depending on the nature of construction capillary transfer of moisture vapourer can can take place and appear in locations away from its initial source.
See. Osmosis .Efflorescence