The water break test is a simple, quick non-destructive test method to evaluate the cleanliness of the to be treated surface. The test is applied to freshly cleaned metals such as aluminium, and other metals that, in a totally clean state, are. The last thing you want on a surface, that is about to be anodised, is a film of any type of residue that prevents that to occur evenly.
The test is based on the concept that the surface being tested for cleanliness is film of water on a clean surface will start out as a continuous sheet which will either dry without any droplets or breaks will show up in the film. A singular defining line which recedes down the surface as the water drains off indicates that the surface is ready to be treated.
If an beading or patching is found;- those areas are not considered 'clean' and should be re-processed before the anodising starts (which is necessary for the right hand side of the metal surface shown below)