Extracting normal maps from a half modelled object


This is a problem that i have never understood, or maybe i haven’t worked it out yet.

Lets say i have a modelled cylinder ventilation shaft with some ring like ridges running through it. I import this to Mudbox, then add some dents with the sculpt tools. when i extract the normal maps, it exports all the dents and all the previously made ridges.

So when i apply the normal in 3Ds Max, it has normal bumps where the ridges are as well as the dents. I intend to keep the ridges in the geometry, i don’t want a bump over the top of them. Only the dents.