Question for a plumber

[Fishboy]

I've just about completed a minor remodel of a bathroom. I've replaced the tile floors, toilet, vanity, and other stuff. Right now, I'm having to move the drain and water lines about a foot to one side to accommodate the new vanity. I took the sheet rock off the wall and had to cut the galvanized (the house is old) drain line above and below the "T" to then insert a PVC fitting to route it to one side. When I took out the old "T" I was surprised to see the buildup of gunk in the drain line. No kidding, about 3/4ths of the drain was clogged with a waxy/old hair kind of gunk that I dug out as far as I could reach. The line is likely in the same shape further down where I couldn't reach, and I'd like to know how I can chemically clean that stuff out.

I have a snake I can run through, and I've had to use it once in awhile when this thing clogs up. Anyone know a product that'll clean galvanized pipe without eating it up?

Aloha pumehana,
Brian and Mary
Lynnwood, WA\Discovery Harbour