Many cleaning specifications call for “Spot Free Drying.” I have always taken this to mean that there should be no visible water (or other) spots on the parts once they exit the cleaning process. The offending spots are usually created when water evaporates leaving behind a solid residue. The resulting residues or spots are cosmetically unappealing and may interfere with the function of the part itself. Furthermore, the spots are often difficult to remove even by manual means once they have formed.
The common way to prevent drying spots is to use water with very low solids content for the final rinse(s) prior to drying. Water with a resistivity in excess of 1 megOhm is usually adequate to prevent spots. There are, however, cases where high purity water alone will not prevent water spots and other cases where there is no need for high purity water at all. It all depends on the part geometry and the nature of the surface being dried.
The worst cases for water spotting are hydrophobic (not liking water) surfaces on which water forms large beads or droplets. Of course, the larger the droplets, the more water volume they will contain. The more volume, the more solids the water will contain with the result that evaporation of a large drop of water is more likely to leave sufficient residue to appear as a spot than the evaporation of a smaller drop even if it has the same concentration of solids. Unfortunately, nice shiny surfaces are more likely to be hydrophobic and tend to form large droplets. Their shininess also enhances the visibility of spots. Orienting parts to minimize the formation of droplets may help, but may not be able to totally overcome the problem of drying spots depending on the three dimensional geometry.
On the other side of the coin, there are some surfaces that are hydrophilic and do not support the formation of water droplets. These are usually “matte” surfaces. Water, instead, tends to flatten out and form a very thin layer that evaporates leaving behind little or no solid residue to form spots. Such surfaces also tend to conveniently mask any spotting effect due to their already dull appearance.
In some cases, visible water spots can be prevented through the use of “rinse aids” which serve to modify the surface tension of water to provide that “sheeting action” for which that “other blue liquid” you put in your home dishwasher is well known. In this case, of course, there is a chemical film left behind but there aren’t any spots because the rinse water spreads evenly and thinly over the surface. The necessity for a distinction between this method and the use of high purity water is obvious! Finally, there are those difficult cases where the required combination of surface qualities and cosmetics are nearly impossible to provide. A steel surface, for example, that needs to be resistant to rust AND spot free may be a challenge best met through the use of a texture or other means since many rust inhibitors do not dry to a spot free finish.
So, when a specification calls for “spot free” drying, one should always research what that really means. There is a difference between a surface without spots and a surface without chemical residues.