When modeling the Williams Tankcars you need to purchase the Walthers 23,000 gal Funnel Flow Tankcars # 932-7250. After you have assembled and washed them with dish detergent, they need to be painted black (Scalecoat II).  I aquired decals from Ron Sheets of the Tanana Valley RR Club (link to site via John's web site).  Using pictures I printed off at John's website to determine decal placement, I decaled the tankcars. After the decals were dry, I sprayed a couple of coats of Testors Clearcote.