Wet sanding primer is the last step for primer preperation, its purpose is to give the primer a smooth surface so your base coat or any other paint your using will look smooth at the end resault. Why use water while sanding? Because this will keep your sandpaper from clogging up too fast and make the primer more smooth.
You will need 600grit wet sandpaper, a bucket of water, and guide coat if you feel you will need it.
It's ok for the water to get primer sludge in it from sanding but try to keep anything else out that may stick to the sandpaper or it will scratch the primer.
If you don't know how much to sand, spray a guide coat over all the area where you sanded with 400grit sandpaper and then sand the guide coat back off.
You don't have to sand one way like if you were sanding wood, you can sand any way you want just as long as you keep your hand flat and dont dig your fingers in to the primer. You can use a small soft sanding block if you think you'll have a problem keeping your hand flat.
If you wanted to paint a brand new car that needed no repairs you can use this step to prep it, just sand the whole car back with the same technique showen in the video and you can spray any type of paint over it, base coat, 2k colour, laquer, ect.
From here on out your primer patches are all ready to be spraypainted. But some of you will need to prep the rest of the panel aswell, go to the next step.