for the letters, get a sanding block and get a few different grits of sandpaper. Be VERY careful around the rest of the paint on the valve cover. Start with maybe 300grit to get the paint/primer off. then repeat on the letters with finer and finer grits (400, 500, 800, 1000, on and on) until the letters look as smooth as you want them to look. you can apply a little metal polish to the letters from that point, and wipe any excess off the paint.
BANG, sexy smooth polished letters. The valve cover would DEFINITELY look a lot better with polished letters.