Surface is what Lightwave calls a material.
In the obj file a part is a group which is identified by the g lines
a surface is a material identified by a usemtl line

the usemtl and g lines identify which polygons are allocated to those groups. All polygons until another g or usemtl are allocated.

the .mtl file provides the parameters for the named material, but only some of them.  

good to see you have it sorted out.

a bug that might still be in Lightwave is the UV information. LW used to discard UV info if the surface was not attached to the uv map in the surface editor. You need to open the texture editor T on the diffuse and specify map type UV and the map name. You dont need to actually have texture applied , just the map.  You can tell if your uvs have been lost by either looking in the obj file for vt lines or open the obj in the free uvmapper utility.