Help! Newbie with Sander marks...How to fix??
Yeah, the previous posts pretty much hit the nail on the head.
If you are staining and clear coating the surface then maybe start with 80-100 grit and keep methodically working the surface while sanding with the grain, and keep your sanding block/tool perfectly flat. Make sure you properly position your lighting to give you the best chance to see any imperfections throughout this whole process. The whole idea is to locate the problem areas and take care of them long before you apply any finish coats. Next, jump up to 150, and then 220. Once you're satisfied with the surface take a rag with mineral spirits and very lightly wipe the surface down. This will temporarily darken the wood and help highlight any imperfections, and it will also evaporate quickly. I have no idea what species of wood you used, or what type of finishing process you have lined up, so the best advice I can give is to make sure that your finishing products are compatible with one another to give you a beautiful, well bonded, and durable finish. Make sure to follow the directions on the can/s of material and don't try and rush it. If your clear coat says to wait 48 hours for your oil based stain to completely cure before applying it, then just do it. After all of that sanding the last thing you want to do is have a topcoat failure and then be forced to do it all over again. Again, just make sure that ALL of your finishing products are complatible with one another for maximum performance and ease of application.
It is also a good idea to very lightly sand between topcoats, and then carefully wipe the entire surface down to remove any dust or particles. If you are using a polyurethane this will help it to smooth out and lay down more flat, especially since your first topcoat will raise the surface grain in the wood. Once the topcoat hardens the raised fibers will be trapped/sealed in it and the light sanding will smooth everything out again. The fibers will only raise once, so any subsequent ultra light sanding between coats will just be to knock off any burrs that crop up for whatever reason. Oh yeah, almost forgot, if you finished out the raw wood with 220 grit as your final sandpaper then absolutely do not use anything any grittier to knock off the burrs between coats. Heck, a 320 or something even a little finer is plenty since you're not trying to remove material, just lightly scuff the surface (to give the next coat maximum bond) and knock off the burrs.
Good luck with your project!