You can use the COOR SURF command described in corman.doc
to compute the solvent accessible surface exposure of atoms, and use an atom selection based on that result.
Assuming the protein is segid A, and hydrogens are to be excluded:
coor surf acce sele segid A .and. .not. type H* end
define surfatm sele prop wmain .gt. 2.0 .and. .not. type H* end
You may need to use a different surface area threshhold (2.0 may be too large or small for your application), or perhaps use a larger probe radius to reduce the exposed area for more buried atoms.