I bought two of the welding rod canisters at lunch today. One by Rod Guard and the other by Lincoln Electronics. The Rod Guard one is a marine blue and just barely not light tight. The Lincoln E. one is red and is def not light tight. I checked by taking the cap off and putting the canister end up to my eye in noon sunlight. With the blue one, you could barely see light transmitted, while the red one let a lot of light through. They could be used as is in a light tight darkroom, or you would have to coat them with something that doesn't transmit light, to use them with the lights on. I will be trying the former method this evening and will post my results.
Oh and the ones I got are not lined with SS. :-)