Hi Mark. One of the reasons you have to sand is that the timber goes fury after you put a coat of clear on. The aim is to get it as smooth as possible. When we do mirror finish surfaces one of the things we do is remove all the shine from the previous coat then we recoat. It has to do with removing the surface imperfections that the brush/roller or spraygun leave behind. That is why you don't use glass paper and finish with wet and dry on the last coat. remember that when you sand the first coat you must be carefull not to sand too much or you will remove the stain. When you get to the final two coats you can sand in small circles if you wish but my experience tells me that you only need to do this with the wet and dry paper. Any scratches that you cause by the sanding as long as they are on the varnish only (not on the stain) will be covered by the next coat. The 120 grit paper is not very rough. If you find that it is a bit rough grab the piece you are going to use and rub it on your driveway for a second or two to smooth it out a bit. If you get stuck along the way or have any further questions let me know. Spraying is overrated unless you have excellent drying conditions. That is why cars are sprayed then they are cut and polished and the ones done in enamel or epoxy are dried in ovens to prevent dust settling on them (which is inevitable). In this same thread type in new telescope and click on the thread with my name on it. have a look at the truss telescope I made and have a look at the leaf at the bottom. if you look carefully you can see a reflection of it. it's not the red paint but the varnish that gave it the shine. I used the same varnish and method I explained earlier. I finally have a topic that I know a lot about, paint/painting.
Regards Cristian