TIN made of polylines - how to convert to a renderable surface?

hi everyone,
i have a TIN mesh created from a DTM and i wish to use it as basis for some visualition within Autocad 2004.

The components making up the TIN are thousands of polyline triangles and i wish to give them solidity and would like to be able to render the ground-surface they would create, only i can't for the life of me figure how or what to do to make them into a renderable surface.

i'm reasonably new to Autocad having mainly used the solid modelling features in laser-scan surveys and thats about it.

we also use microstation SE at our work which holds my triangular mesh as thousands of shapes which do render without a faff but I find the 3D modelling tools and pants UCS feature within microstation un-intuitive to use.

any help gladly appreciated,

matty

ok, i've come across the REGION command which seems to give the triangles solidity, they are rather jaggy though.

is there a way to combine all the triangles into a whole surface so phong shading would create some smoothing??????

cheers...