Once done with the sig, make a new layer on top.
go to select>select all
then go to Edit>Stroke
Make it 1px and black or white or whatever color you want
Push OK and Cntl+D and your done.
another way is to create new layer, select | all, select | modify | border, pick pixel size then fillin with color of choice. I find this way gives a nice feathered look.