The answer to this question is very dependant upon what the coating is and how well it's attached to the underlying substrate. The ultimate strength of the overall joint (i.e. substrate/coating/adhesive) will be dictated by the weakest interface or material. If the coating is well adhered/attached to the surface, and the substrates are stronger than the load applied, then failure will occur either within the adhesive (cohesive failure) or at the adhesive/coating interface (adhesive failure). Without specific testing using representative loading conditions, it is not always possible to predict the outcome.
At the very least, the coating surface must be clean and free from contamination and particulate debris. This can be achieved by carrying out a simple degreasing operation using either a solvent or detergent system, followed by drying. If adhesive failure is still seen then the coating should be lightly abraded to roughen its surface without damaging the coating integrity. Where there is an indication that the coating is more weakly adhered to the surface, it is better to remove the coating altogether in the area to be bonded. If this is not possible then an adhesive system with a lower modulus and greater strain-to-failure should be considered, as this will have the effect of reducing the peak stress at the joint edges, at the expense of lowering the overall joint stiffness.