Background The aim of this study was to compare the flexural strength and Vickers hardness of tooth‐coloured restorative materials with and without applying a self‐adhesive coating for up to 6 months. Methods Specimens were prepared from three resin composites (RC), two resin‐modified glass‐ionomer cements (RM‐GIC) and two conventional glass‐ionomer cements (CGIC). All materials were tested both with and without applying G‐Coat Plus (GCP). Specimens were conditioned in 37 °C distilled deionized water for 24 h, and 1, 3 and 6 months. The specimens were strength tested using a four‐point bend test jig in a universal testing machine. The broken specimen's halves were used for Vickers hardness testing. Representative specimens were examined under an environmental scanning electron microscope. Results Data analysis showed that regardless of time and materials, generally the surface coating was associated with a significant increase in the flexural strength of the materials. Applying the GCP decreased the hardness of almost all materials significantly (P < 0.05) and effect of time intervals on hardness was material dependent. Conclusions The load‐bearing capacity of the restorative materials was affected by applying self‐adhesive coating and ageing. The CGIC had significantly higher hardness but lower flexural strength than the RM‐GIC and RC.