Hi Steve , using two scratch coats is a good idea it will keep the thickness of the coats down to a min,it would be a good idea to rule off the 2nd scratch aswell this will help when it comes to your top coat. My main concern would be to seal the old dashing up if not painted ,for this i would use a SBR slurry one coat 24 hours before you start rendering and one coat as you start your scratch coat just make sure its not completly dry as you cover it will also give a create a very good bond for your 1st coat, I hope this helps you Church.