The present invention provides a primer for a thermoplastic and/or elastomeric substrate, the primer comprising a film of an epoxy-modified polymer selected from the group consisting of an epoxy-modified-thermoplastic polymer, an epoxy-modified-thermoplastic polymeric composite, an epoxy-modified-elastomeric polymer, an epoxy-modified-elastomeric polymeric composite, a blend thereof, and any mixture thereof. Also provided are methods of repairing, inserting, assembling and coating a thermoplastic or elastomeric substrate using the primer of the present invention.