[MeanPi2]

"sections of a model"? As in sections of a whole single model? You could chop each 'section' you want to appear indepently of the others into a seperate surface so you can envelope (in Surface Editor) each 'sections' transparancy independently like Chrispo says. There could be potential problems with surface consistancy across sections but should be fixable if it shows up. Then you'd mess with the keys in the Graph Editor when you hit the 'wee e' in Surface Editor.

You could also, for some reason, chop the model up into seperate objects, parent all parts to a null and mess with each objects dissolve envelope in 'object properties-render'.

And the last idea (but certianly not last way) is creating a custom transparancy map, or series of maps- but not really sure how that would work.