"Surface preparation" and "substrate treatment" (substrate adjustment) are often used with similar meanings, but they refer to different tasks in precise terms.
"Substrate adjustment" is the process of transforming the "substrate," which has not been treated with paint or other finishes, into a surface suitable for painting. "Undercoat treatment" is the process of transforming the "undercoat," which has already been painted or treated, into a surface suitable for painting. Both involve the work of "changing the surface of the object to a state suitable for painting," but the conditions before the work is performed are different. Additionally, substrate adjustment is fundamentally based on surface polishing and degreasing, while undercoat treatment also includes the repair of deteriorated surfaces with primer paint or putty.
However, in practice, both "substrate adjustment" and "undercoat treatment" are often listed in specifications without distinction! It may be better not to overthink it and to understand both as having the meaning of "transforming into a surface suitable for painting."
For more details on "Methods of substrate adjustment and undercoat treatment," "Importance of substrate adjustment and undercoat treatment," and "Standards for substrate adjustment and undercoat treatment," please refer to the related links below.
