I bought an indented firewall gas tank that looked clean when shining a light in the gas gauge hole and the filler hole. I thought I was very lucky to find such a good tank but I wanted to make sure so I took my 4" hole saw and cut a hole on the firewall side of the tank above the outlet pipe and what a surprise! There was the biggest rat's nest you ever saw. It couldn't be seen because of the baffles. Since I had to weld up one hole I cut three more. Two more on the bottom of the tank and another on the firewall side of the tank. I was able to see the spotwelds of the baffles so I placed these 4" holes so I could sandblast into each compartment and of course cleaned out the rat's nest. After welding up the holes I had my radiator man pressure test the tank which checked out. After some grinding of welds, some body filler and primer you can't tell where I cut the holes. This ended up a lot of work but in my case where I did not know the history of the tank, it was worth the effort.