Sanding, UGH! I know everyone's favorite part of prepping. Just put your big girl pants on and do it! Whether you use an electric sander or you sand by hand, you won't regret it.
Why do you sand?
The coatings on your piece are likely not meant to allow other materials to adhere to them, so the only way to accomplish a surface that will allow primer/paint to adhere to it is to sand.
What is scuff sanding?
Scuff sanding is a light sanding to the surface of your project. It helps smooth out imperfections, and gives paint or varnish something to adhere too. You should also scuff sand in-between coats. Why? Scuff sanding will allow you to knock down any small imperfections like dust or fine hairs that may have landed on the drying surface and allows you to achieve a smoother finished surface.
Choosing the right sandpaper.
When sanding bare wood you will need a sand paper grit from 120 to 150.
Scuffing between coats of paint or a sealer a sandpaper grit between 180 and 220 are best.
If you are sanding between coats of varnish and are looking for a high-gloss finish a 320 grit sandpaper is best suited for this project.
Sandpaper with a grit of 600 to 800 is best for high-gloss varnish, polyurethane, and lacquer. Sanding this type of finish is a little different, you will need to wet sand to prevent scratches in your finish. Wet sanding is applying water to the surface you are sanding, you can do this by using a spray bottle to wet the surface and then gently hand sand with your 600-800 grit sandpaper.
Do not skip the sanding, even in between coats. This will always put you a step above the rest.