No, it is not safe to use a rusted propane tank. Rust can weaken the tank's metal, leading to leaks or rupture, especially under pressure. A compromised tank poses a serious fire and explosion hazard. It should be inspected by a qualified professional or safely disposed of.
You can get a rusty propane tank inspected at authorized propane dealers, certified tank requalification facilities, or some hardware stores that offer propane services. These professionals have the expertise and equipment to assess the extent of the rust and determine if the tank is still safe for use or if it needs to be condemned and replaced. Always choose a reputable and certified service provider.