Category Archives: Walnut Shell

Picking Grit Size For Sandblasting

How do you pick a grit size?

A quick and simple answer is to pick a maximum grain size that is about equal to the thickness of the coating being removed.  So if what you are trying to remove is 1.6 mm thick (.063 inch), you would choose a 12 grit.  A bit finer size is often used as well, because you get better coverage as the finer particles fil into grooves, cracks, voids in the surface that the larger particles will not fit into.

A more detailed answer involves selecting a grit size based on the surface profile you are attempting to achieve on your substrate.  A quick guide can be found here.

Keep in mind that the more rounded a particle is, the more contact area it has with your substrate, and this can speed up cleaning rates.  Glass bead is a good example of a cleaning abrasive, whereas the more angular and blocky aluminum oxide particle is useful for actual surface preparation.  (Where you want the abrasive to leave a surface profile for a future coating).




Surface Preparation vs Surface Cleaning

Question:  Are You Blasting for Surface Preparation or to Clean A Surface?  The difference between the two is critical for success.  Lets quickly look at them one at a time.

Surface Preparation:  If you are blasting for surface preparation, you are trying to remove all contaminants, as well as prep your substrate to receive a coating.  This means going to the manufacturer of your coating  to find out the ideal surface profile for the coating you are going to use.   Too much profile will result in premature coating failure after the liquid in the coating dries and shrinks, exposing the peaks of the surface profile.  Too little of a profile will also result in premature coating failure, causing the coating to essentially peel off early, as the coating cannot get sufficient “hold” onto the surface.

This is why many painters will buy paint only from reputable paint companies, as they have the technical data on their coatings and can quickly tell you what your surface profile needs to be for maximum adhesion and lifespan.  The percentage of liquid to solids in any given coating will dramatically affect how much a paint shrinks when dry – this is not the time to guess!

Commonly purchased abrasives for surface preparation often include aluminum oxides, crushed bottle glass, coal and copper slags.

Cleaning Preparation:  Here the goal is to simply clean the surface without changing the substrate.  Often abrasives like glass bead, walnut shell, or plastic grit can be used in such cases, depending on your grit size, pressure, angle, etc.