I am pretty sure I've tried that brand. I find hardware store epoxy putties too hard to sand. Maybe you could try mixing in some microballoons. I personally use Superfil. Microballoons are pre-mixed in there.
PC-7 is gritty and will leave a rough surface to fillets, and it is hard to spread evenly - although I have not blasted it with a heat gun while trying to draw a fillet. It probably has some applications, but I think you are better served by a number of other products for fillets. However, if you are going to cover it with tip-to-tip glass or carbon, the gritty surface would probably help with adhesion.