do I really need to sand... before priming?
Ideally yes. The role of sanding is not just about smoothing, it can also be about prepping a surface to properly accept a finish (or glue).
Any older wood surface should be sanded lightly prior to applying glue to ensure a good bond and the same holds for when paint or varnish is applied. Just to be clear, you can get away with not doing this in some cases but with waterbased products in particular it will give better security if you sand.
This shouldn't be any kind of heavy sanding, just a few passes with a med-fine paper is more than enough. If using a random-orbit sander finish with a pass sanding by hand (using a block) in the direction of the grain, using the same grit as fitted to the ROS.
The raised grain is a separate concern. Because the primer is raising the grain pre-wetting to raise the grain can be advisable. After this knock back the raised grain with just enough sanding as needed to remove it and no more.
So in summary here's the prep procedure you might adopt:
- sand lightly overall with 120 to 180 grit, no need to go to finer paper than this when painting;
- dampen the surface with warm water to raise the grain;
- let the wood dry thoroughly, a couple of hours if necessary;
- sand very lightly along the grain to just to remove the raised grain, using a finer paper such as 180 to 220 grit;