To a silicone resin obtained through cohydrolytic condensation of 100 pbw of a mixture of 1-50 Si mol % of tetraalkoxysilane, 50-99 Si mol % of trialkoxysilane and 0-10 Si mol % of dialkoxysilane with less than 100 pbw of water, having a Mn of at least 1,500 and containing 30-80 mol % of R.sup.1SiO.sub.3/2 units and 20-70 mol % of R.sup.1SiO.sub.2/2(OX) and R.sup.1SiO.sub.1/2(OX).sub.2 units combined, a curing catalyst and a solvent are added to formulate a silicone resin composition. It is applied and heat cured to a plastic substrate to form a protective coat having a high hardness and improved properties such as mar resistance, crack resistance, adhesion, water resistance, heat resistance and weathering resistance.