Sorry for the delay! I did my first layer of quikcrete on Wednesday. It was harder than I expected- apparently I didn't sandpaper the GS enough. The cement stuck well to the rougher portions, but for the rest it was difficult. Because of that (and because I used a thin layer) there are some pieces that have chipped off. I'm going to put silicone on top of the cracks and chips before doing my final layer of cement.
Anyway! Here are some much needed pictures:
Full tank shot
In this one you can see some cracks and chips. I would do two more layers of cement, but I'm afraid that would make it too heavy and it would pull off the styro. So I think silicone patches + cement is the best way to go.
I'm mixing up my "substrate" to be pressed into the final layer of cement. I'll probably be doing that later tonight so the silicone has a chance to cure a bit.