Sift together flour, sugar, soda and salt.
Place in a large bowl with the whole wheat flour.
Make a well in the centre and add the buttermilk.
You may need to add more than 2 1/2 cups.
Mix enough to make a soft elastic dough.
Knead it lightly into a ball in the bowl.
Turn out onto a greased baking sheet and pat into a circle 1 1/2" thick.
Mark an X 1/4" deep with a floured knife.
Brush surface with sweet milk.
Bake at 425 degrees for 25 minutes.
Reduce heat to 350 degrees and continue baking 15 minutes more or until bread sounds hollow when tapped.
Remove from sheet and cool on a rack.
It is best if left 4 hours before being sliced.