For timber surface preparation, you may need a power sander. Use an orbital/finishing sander to run over and smooth out rough areas after using the grinder. For this you will require safety equipment including glasses/goggles, earplugs, respirator and a suit to keep you clean. Start using the low setting on the sanding machine and fine grade paper. (a rough grade or disc will possibly damage your timber).
After sanding is completed, apply primer to the surface so you can see what filling is necessary. Nail holes etc are best fixed using linseed oil putty. Use acrylic gapping compound for gaps in the end of boards, use gapping compound (silicone based has poor adhesion for paint.)
If done effectively the preparation process will give your paint work over ten years of life.
For a combined approach, you might like to have a professional prepare your surface first, and then just carry out the painting yourself.