I want to attach about 10 foot long 2x4's horizontally to my wall. I do not want brackets showing. How can I do this securely. I need to do it in several areas of my house. These will be used for decoration. How can I do it. Thanks.

We are installing all new baseboards throughout our home. We just began cutting the other day and we've already run into significant expansion and contraction of the wood between cutting it and actually installing it (we cut the pieces and they were a snug fit but now that they're nailed in and I'm ready to touch up the paint, there are gaps at the corner). I'm assuming that the wood will continue to expand and contract regularly over the years so I'm hesitant to fill the gaps with anything too solid. Should I be concerned with leaving the boards some flex room and use a fleixble filler (like caulk), or will they be fine if I put in something like wood putty or a sawdust/wood glue mix? Laurie