>2. In all my reading I did not take in the fact that
>so much sand was added to the final painting layer, I
>thought it would be almost pure lime...a smooth
>painting surface.

Lime putty by itself has no strenght. what keeps the plaster together is the sand, lime adheres to the sand and so on so on, just like glue...
The smooth painting surface is achieved by troweling...
The smooth part will be a "thin layer" on the top what is known as "skin" that is a result of passing the trowel By "thin layer" i mean microns not even milimeters. (Over-troweling, however brings out "fat" - you will see it collecting on the edge of the trowel - you will need a rug to whipe it off before the next pass of the trowel, do not over-trowel it weakens the plaster - everything is good in moderation)

You perhaps missread the source or what is more likely it was written by the person that had no idea/experience of painting in fresco, i have come through hundreds of such books, in fact about 95%.

look for the old book (1947 or so) by Olle Nordmark
(american artists group, inc)

(however, nothing beats hands on workshop - you would be amazed how clear it becomes. we bell be teaching 2 in may and 2 in june go to http://frescoschool.com exact dates will be up in a couple of days. or call 310-337-2783)