Can you use primer as a finish coat?

You don't specify on what material you are using the primer on.
In either case primer is paint that is made to seal and bond to the material you are painting, which in turn helps the regulat paint to bond. You CAN use primer as a final coat but the primer won't give you the hardness that a paint will or the shine, but there is no reason that you can't use it. Depends on the look you want. Hope this helps