The smooth part I just used fibreglass to repair cracks and missing pieces, you have to grind out the cracks a bit, i also roughed up the underside of the cracks and added fiberglass resin and reinforced with (gorilla hair) mat. Really strong and then you just finish with some bondo, primer then paint interior color. One of my panels on the botom part (grain) had a crack that was very fine so I justvreinforced it from back side and re dyed in semi black and is gardly noticable. I have a set of trashed panels that I molded in 3 component speakers with fleese and fiberglass resin and painted the bottom (grain side) with a speckle paint that gives it a rough surface and resembles grain, then dyed it black, had tons of compliments on how it was done.