The type and size of blasting abrasive used have a significant effect on the blast profile or amplitude produced. Whilst most people understand that blasting with media will remove previous coatings and rust it’s also worth considering surface roughness. When applying a paint system to freshly blasted steel it’s best to ask if the person blasting is using a mixture of shot and grit. This blast media will remove old coatings and rust whilst producing a surface roughness that the paint will adhere to much better than if not used.
The Image of the right shows what all of us will see with our naked eye, rust and old coatings removed and a nice clean surface. The image on the left shows the surface roughness, you can now see peaks and troughs and a jagged surface profile, this is perfect for paint however how high are these peaks in the metal??
If the maximum peak height of the metal is 300microns a paint system of 270microns isn’t going to cover these rogue peaks, therefore more paint is required. If these peaks are not covered, they’ll pretty soon start to rust.
This will leave your freshly finished surface with spots of rust across the paint film, ever had this??
Above is my very basic steel profile, you can see on this that there are 4 peaks above the yellow line, these are the rogue peaks. If this yellow line represents our paint thickness, we will have areas that will rust as they are above the paint film.
Whether this is a car restoration or painting the newly blasted gates at home this applies.