to wetsand prior to paint. 180 is way too coarse and will require too thick of primer to fill the sanding scratches. You could use 220 to remove lots of material if you are concerned about film thickness then follow up with 400 or 600 prior to paint. Use a sanding block or you will get waviness from irregularities of your hand. Blocking will take off the high spots first and you can stop when the whole surface is matte finish. Sand in diagonal strokes across the whole panel at a time work both right and left handed to get a criss-cross effect.