Longboards work best when move it at a small angle relative to its length, instead of just sawing it back and forth in a straight line. Here's a tip for sanding with a longboard before apply varnish: hold the longboard at a 20 or 30 angle as you move it in the direction of the wood grain.
However, if you are sanding primer, try to sand with the longblock in one direction, so the scratch marks have the same orientation. Choose a different direction every time you change the paper on the longboard to a finer grit. Another tip for extending the life of the sanding paper and the efficiency of the long board is to use a metal or wood rod to knock dust, paint, primer etc off the paper to keep it from clogging.
A sanding block is used to hold sandpaper. At their most basic, sanding blocks are wood with one smooth, flat side. Sandpaper is wrapped around the block, and held in place with your fingers, adhesive backing, clips, teeth, or clamps. Hand sanding blocks improve the efficiency and quality of any hand sanding project because they evenly distribute sanding pressure, whereas direct hand sanding without a block can yield uneven results due to finger pressure. It is a good rule of thumb to use the longest block that will fit in a certain area you're sanding. Most hand sanding blocks hold a 1/4 (quarter) sheet of sandpaper.