Hi LSETER, When I painted my AT AT (I know what you guys are thinking) I didn't prime it. I rubbed down the surface with an abrasive pad not unlike what you find for washing dishes. I've found on most of AMT/ERTL kits if the surface is properly cleaned Testors enamels work and hold fine. This is not the case for all kits, Monogram, Revell, scratch projects ect. will get a coat of spray can primer. My preference is Now brand, for primer only, as it is fast drying and rubs out smooth.
For the AT AT, and would also be the case for the AT ST, I sprayed on Testors Light Ghost Grey (looks more white with a hint of grey) and detailed with Dark Ghost Grey. After that set I dry brushed with white to bring out even more detail and tone down the DK Ghost Grey. The weathering was done with grey and black pastels and a hint of rust here and there. The only thing to decide on with the extent of weathering is if it was on Hoth (less) or Endor (more) I went with the Hoth look.
This is of course only what I did and I am by no means an expert just really really lucky.
Hope this can help in some way.
Matt