I don’t think it’s 304 grade SS. It may be 403. Genuine 304 SS would never get oxidised or rusted except in some certain conditions. We do SS letters all the time, 99% for outdoor and we only use 304. We never had such issues.
Some companies use 403 instead of 304 when it’s for indoor just because it’s slightly cheaper. But if the amount of moisture is high in the room then 403 ss will start having such issues.
Having said that, I’m not saying that it’s definitely 403 but it’s likely.
<font face=”inherit”>If you could possible take some closer photos, I may </font>understand<font face=”inherit”> better what it is.</font>