Anyone have any suggestions on how, or what chemicals, and/or products I might be able to use to treat the inside of a custom-made steel gas tank? thanks, Kerry

Try searching the web for a product called POR-15 it is used in the marine industry

http://www.theshopproducts.com/dionts.htm
Try this

http://www.por-15.com/tankseal_fuelpreserve.html
Or this

Thank you everyone for your responses! I'm familiar with the POR products. I checked the web sites that you supplied, and they were very helpful. Thanks again, Kerry