What is Adhesion
The ANZ Standard 2310:2002 Glossary of paint and painting terms defines “Adhesion” as the sum total of the forces of attachment between a dry paint film and its substrate.
Therefore when paint loses its adhesion it can:
- blister (deform the paint film arising from the detachment of one or more of the constituent coats/layers)
- check (break the surface of the paint film but not visually show the underlying surface);
- craze (forma minute criss-cross cracks on the surface of the paint film);
- crack (form defined breaks in the paint film which exposes the substrate);
- flake (the complete detachment of pieces of the paint film from the underlying surface or substrate)
- peel (localised loss of adhesion between the paint film and substrate)
Image 1: Blistering Paint Image 2: Cracking paint
Why does paint lose adhesion?
When re-painting existing painted surfaces during repairs we sometimes come across wall and ceiling linings that have not been previously prepared correctly. When the new paint is applied this adheres to the original paint layer but not to the original substrate ie plasterboard etc. If the substrate has not been prepared correctly it is unreasonable to expect the new paint surface will adhere successfully.
Fit for Purpose
As soon as the painting contractor paints over the existing paint layer he/she accepts that substrate as “fit for purpose”. This means if the paint is to fail in the future ie blister, craze, crack, flake or peel, it is the responsibility of the painting contractor due to the poor preparation of the substrate.
How to Test for Adhesion Loss
An adhesion test is a method of testing the substrate/paint quality.
The Resene website explains an adhesion test as follows:
Thoroughly clean the test area. Use a sharp razor blade or putty knife and cut a small ‘X’ shape on the surface in the area you wish to test. Carefully apply half of a clean 4cm strip of tape to the cut area. Ensure the tape is firmly adhered by pressing it down with your thumb nail. Hold the free ends of the tape at right angles to the test surface and yank it off with a sharp pulling movement away from the surface. Observe the removed tape for old paint and stain. If paint is easily pulled off, the adhesion of that coating must be considered suspect.
Peeling is an adhesion failure whereby the paint film peels away from the surface. There are two types of peeling:
- Total film failure back to bare surface (all coats are peeling);
- Intercoat failure (one or more coats separate from a lower coat).
Conclusion
It is the responsibility of the painting contractor to ensure that the substrate is fit for purpose by completing an adhesion test. This test should be documented and photos taken before and after the test and also of the tape used.
Source:
Resene Website – Peeling Paint