I have mostly always been disappointed with the bottom side of my aluminum castings. The sand has been on the fragile side. As an experiment I tried using wetter facing sand. It worked.
The 3D printed pattern.
The cast with the sprue and gates/
After it was cut.
A close look at the reproduction of the features on the 3D print.
OK yeah I see the lump too but this is the bottom side and but for the lump it is very good.
I tried smoothing this with a propane torch. The top sunk. Again you can see the pattern in the 3D print. The face you see is the bottom. It looks nice too.