I'm painting my kitchen after some hired help put up drywall and skim coated. They sanded the wall, I cleaned up and wiped the walls clean of dust.
My contractor said that I should prime, sand with 100 grit before I paint.
What is the reason and benefit of sanding after priming?
My father-in-law does cabinetry and explained it to me like this:
When you paint, the paint has to have something to hold on to. So, for instance, if you just slapped latex paint over old lacquer, it likely wouldn't hold very well, if at all. Primer helps by bonding to the old surface and being bondable to the paint.
Sanding cuts holes into the surface. It increases the primed area for the paint to hold on to and improves the bonding. I can speak from experience when I say it really does help.
100 grit is a bit much tho. I would suggest something more like 220 (most sanding blocks come around this grit). 100 would be if I were priming something else. For instance, I had an old bed frame that was just stained and lacquered. I sanded it with 150, primed, sanded 220 and then painted a final coat. It was very nice after I finished.
Sanding after applying primer should help keep your surface as smooth and flat as possible, eliminating brush marks, or extra little globs.
Also when painting, like others have pointed out, can raise little fibers from the drywall, going over the surface with a sheet of sandpaper can help get rid of those lose fibers.
I usually sand between each coat of painting/priming (though most paints now come as paint/primer combos.
From my knowledge sanding lightly is ok after priming new drywall, but manufacturers suggest only sanding a light first coat. The reason behind this is that primer is only good for about 7 days to be topcoated. Primer is sticky within that time to take paint.
If it's a high-build primer in an aerosol spray can (more for automotive uses) then sanding will it will help the primer do it's job to fill in the little pin holes and scratches. This way the primer can perform like a really thin skim coat of auto body filler or spot putty.
Sanding after priming will help you eliminate bumps and dents to achieve a better finish with the paint layer.
Primer raises burnt paper from sanding the drywall. This burnt paper is not detectable to the eye prior to priming, and cannot be removed until after it is primed. It will feel like there is sand on the wall over and under the seams and next to the butt joints. Simply "brush" over it lightly with 120 or 150 and it falls off.
