Wet-sanding primer, intermediate and finish coats is a process that is time-consuming and tedius, but of utmost importance. Your job can be made easier by obtaining good tools (sanding blocks, etc.) and high-quality sandpaper, but ultimately you're the one who makes or breaks the finished surface.
If you've ever wet-sanded before you already have a feel for how the surface resists the sandpaper as you slide it over, and only practice can teach you how to sense when the paper's grit is spent and it's time to replace the sheet.
We can't help you there, but we can give this great little tip. Rather than using a running hose to keep the surface wet during sanding, use a bucket of water with about a teaspoon of liquid detergent added to it. The detergent is a surface tension reducer, so it will cut the friction between the sandpaper and the surface, making the sanding process a little less hard on your muscles.
Not only that, but the detergent helps float away the sanded particles, so the paper lasts a little longer without "loading up." Try it; you'll be glad you did!