Aluminium Metal Spray (AMS) (thermal spray) coating was applied onto steel substrates and overcoated with an epoxy/polyurethane paint system. Test panels were exposed to neutral salt spray test for 1500 and 3000 hours. Premature failure of the coating system, involving blistering of the paint, was observed which was attributed to the mechanisms of volume expansion and cathodic blistering due to Al(OH)₃ formation as a result of aluminium corrosion under the paint system. Electrochemical Impedance Spectroscopy was conducted to evaluate the performance of AMS with and without epoxy sealer and to understand the effect of porosity in AMS coating. The results showed no significant performance improvement due to sealer use and a poor corrosion resistance of the AMS compared to commercially pure aluminium which was attributed to porosity in the spray coating. Thus, the higher surface area of the porous coating gave greater opportunity for corrosion the case where high amounts of Al(OH)₃ were formed in the pores in contrast to tightly adherent continuous oxide layer formed. The consequent volume expansion  (due to Al(OH)₃ formation in the pores) degraded the weakly bonded spray coating which exposed the rest of aluminium and led to further Al(OH)₃ formation and de-bonding of the paint.