Rider, is the garage dashed or plain faced, do as richard said with the joints, then fully mesh the whole unit in your baserend, i would use an high polymer render if poss mate, if your mixing your own then nice clean plastering mixed at 4:1 using a gauging solution of sbr /water mixed at 4:1 and an handfull of fibres if poss, top in weaker mix. you can modify mono in the same waywith the sbr/water mix as well if you wish, and obviously it will need priming/scudding if theres no key, and there is the option that goody adopted lath and aquapanel, base ,mesh, and top in prefered finish