Backdrops are good, sky-colored backdrops are better. Even better yet are sky-colored backdrops that don't have large wood grain in them like can be seen in this NTrak module. The ripples will stand out quite prominently in photos using small apertures (which means a higher f-stop number and therefore larger depth-of-field like this image). There are a couple of ways around this problem, with the most simple being to use a smooth material other than wood as your backdrop material. Styrene sheet is available in large sizes up to 4'x8' at specialty plastics stores, and if you don't have a plastics dealer nearby, your local hardware store or lumberyard may be able to order it for you. Another option is to cover the backdrop with another material, which could include any of the preprinted backdrops that are easily available at almost every hobby shop, photos cut out of calendars from past years, or printed from your own printer on photo paper. The key to any backdrop cover is that it needs to be thick enough to hide the wood grain beneath it, so papers should be somewhere around cardstock thickness.