Cob: Counter-attacks

[Frances Grill]

    Greetings, Boy Charmaine, you never cease to amaze me! I'd have to
quit ranching to read as much as you do. This inverted counter-attack 
does leave me with a few questions, Troweling has several purposes as 
you well know. Obviously it serves to flatten the exposed surface. It 
also serves to draw cement and water up through the aggregate so that 
all the interior surfaces and spaces get covered with bonding agent 
(cement in most cases). Stone or aggregate gets pushed down and cement 
comes to the surface to form a sort of monolithic plane.As the concrete
sets, each pass with the finish trowel "burns" the surface to proper
glass like hardness...burnishes the surface just as a rub on clay 
plaster would. So with none of this happening on the underside, how does 
one get a smooth non-porous surface? Even micro pores are going to store 
bacteria no???What am i missing, Besides a copy od Fine Home Building ? 
 PAZ,Pedro