It's been a long time since I've used a paint gun so I can't comment on product specifics of new materials BUT the basics should be the same.

The required "smoothness" of the finish of the base coat prior to topcoating depends on the requirements of the topcoat. The sanding serves two purposes. One it creates a more level surface, removing high spots for a smoother topcoat. Second it creates grooves in the surface which increases the surface area for the topcoat to bond too, increasing the "tooth". It may feel smoother but if you could look at it under a microscope you would see them.

On the old acrylic lacquer I shot I couldn't use anything rougher than 600 grit or the sand scratches would swell and show in the topcoat. The enamel we shot would peel if we used anything finer than 400 (preferably 320) because there wasn't enough "tooth".

Paint is nothing more than a pigmented adhesive. All adhesives I'm aware of require a roughened, etched or sanded finish to bond correctly.

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