Hoping to salvage a poorly sloped shower pan

I pointed out some dips in the corners to the guy who built my shower pan and his action was to add more mixture in the corners and drag it down. He did not smooth it out, or "screed" it, I believe is the correct term, so I'm left with a situation where the new mixture ends very roughly on top of his first attempt. He has stopped work for now, while I save up some more money to finish the job (and more than likely hire someone else), but I'm wondering if I can use my grinder to smooth it out, and put another topping on it and try to screed it properly this time. I'm hoping to salvage this and not have to jack hammer this shower pan up.