"Phantom" mask.... thickness went wonky??

[talisman018]

Hi everyone!

I was tooling around in Amapi 6 and tried to make a simple "Phantom of the Opera" mask for Poser's Victoria figure. What I did was this:

1/ imported her head as an obj.
2/ boolean cut away as much as possible from the face.
3/ boolean cut the remainder in half.
4/ carefully deleted faces for everything I didn't want as part of the mask. (Getting rid of the tendon hole in the cheek was harder than I thought!)
5/ cut down the number of faces with subdivision.
6/ applied a tiny amount of thickness.

This is where it all went wrong. It was perfect until the thickness tool went and made it suddenly covered in spikes and buckled it in spots. I only wanted a slight raising so it wouldn't look like it was just painted on.

What did I do wrong? If I tried to make clothing using this method, wouldn't this happen all over again?

Thanks for looking!