Sorry, Bob, I can't figure out how to get the images on this post. Follow the URl and you will find a picture of my two favorites for this.
http://www.zooomr.com/photos/zippyzingo/The top one is dark from heating it over a flame. It is very thin but not sharp. Technically, it is something called an"Alginage spatula", used in the dental trade to mix some of the materials used in moldmaking. It's thin and very flexible. You can see that it is also a bit curved. It didn't come that way but it works very well like this, particularly when working under a bridge or finger board.
The bottom one is a plaster knife from a dental supply house that is made just a bit thicker so not quite so flexible. Either one could be made from a putty knife. The hardest part would be thinning them out must a bit without ruining their flexibility and temper. (just don't overheat them if you grind them down a bit. )
I found a link to a dental lab supply company that carries examples of these sorts of spatulas.
http://www.pearsondental.com/catalog/product.asp?majcatid=614&catid=4753&pid=178... I forget to take a picture of a small square putty knife that I sometimes use, heated in a flame, along with the round headed spatulas. Is just a wooden handled putty knife with the corners rounded a little so they won't dig in, and the edge thinned down a bit. Sometimes you need a straight edge but I think the round knife is easier to handle, particularly when just starting.
ZZ