So i got the post out but theres some icky lookin stuff in the bottom of it..grease...13 years of accumulated crud and so on. What do you recommend for
A. Cleaning it out, and
B. Protecting it in the years to come against rust
I do not know whether your FAT has a hole from BB to Seattube, but if so you could remove quite a lot by using degreaser and bended iron thread. It is the way how I removed the ML from my BB. I got it entirely clean. (I than applied Mike Sander, but that is another story)
On several other bikes I used an aerosol can Fluid Film AS-R. It is more solid as the tested Liquid A. It is solid enough to not wash away and unlike waxes it keeps crawling forever and it doesn't dry out. I am pretty sure for bicylce use you can't have anything better.
The prefered method is mechanical rustremoving. In case place very difficult to approach or for fighting last tiny rustparticles and creating stable underground for further treatment (important) you could go for chemical rustremoving.