If the image doesn't stamp as evenly and as completely as I would like, I use an aquapainter to fill in and smooth out the image.
I use an EMPTY aquapainter and dip the tip into water and squeeze out the excess water with my fingertips. You want the tip just barely damp. I pick up the ink on the aquapainter and then fill in the image. If the brush is too wet it will look smeary. If the brush is too dry it will have brush marks.
Play with it and see. Once you get the hang of this technique, it is easy to get the image looking just the way you want. HTH