I've looked into it. I decided that I would much rather buy smooth ones and paint them. I know of members that have smoothed the texture, but that was before smooth factory panels were available. IMHO, less time spent sanding is more time spent on other mods.
I think the only one with the smooth paintable rockers was the SS. I sanded, smoothed, and painted mine. It was a ton of sanding. The SS ones were hard to get at the time, probably not to hard to find now.