Another dedicated Tripp fan here. I have sent 6 pistols his way, and all came back perfect. He's also a super 1911 pistolsmith if you want checkering, magwell, etc. while it's there.
My experience with NP3 on externals was not good. However, for internals on a rifle bolt, NP3 cannot be beat!
Be aware that hardchrome is impossible to do on aluminum or scandium/aluminum frames -- your best bet is hardchrome the steel and coat the other alloys (e.g. Cobra Coat, CCR's DuraCoat, etc.). I have several Sigs in this configuration, and they've all held up very well.
Rich