Can I use this in an outdoor wood burning fire pit?

Thank you for the question but you did not provide any background regarding the actual use making it difficult to provide a proper response. The Loctite Extend Rust Neutralizer is a rust converter. It can be used on rusted metal to convert to a paintable surface. However, since you mention a fire pit keep in mind that this coating has a service temperature up to 200°F so it may not hold up. Last it should not be used on any type of cooking surface. Hopefully this addresses you question.