One way to do it is to use stencil mask. Something along the lines of:
http://www.eastwoodco.com/shopping/...34&iSubCat=1348&iProductID=14463Once you paint the sign, leave the masking on and do whatever you need to do to distress the paint. This helps to protect the surrounding paint from damage. I would paint a couple of thin coats, let it dry and use fine steel wool or thinner or whatever works to get the aged or distressed look you want.
Let that cure somewhat and then strip the stencil mask and ease the paint edges.
Whatever you decide to do, it is always a good idea to try it on a test panel before comitting to the door.
