A finishing fragrance is not a substitute for actually dealing with whatever smell was already in the car before you started, and treating it that way is a mistake worth avoiding early. Layering a good scent on top of an unresolved odor problem, smoke, pet smell, mildew from a slow leak, does not solve the underlying issue. It usually just muddies it into something worse.
The right order of operations starts with actually identifying and addressing the source. Interior extraction, ozone treatment for stubborn cases, replacing anything genuinely contaminated, whatever the specific odor calls for. This is a separate service from the finishing fragrance, and it should be treated as one, both in your process and in how you communicate it to the client.
Once the underlying odor is genuinely resolved, the finishing fragrance does the job it is actually built for: giving a clean cabin a considered, pleasant scent identity, rather than trying to mask something that was never fully addressed. This is also the only scenario where the fragrance will genuinely last its full 2–4 weeks the way it is supposed to, since a competing odor source will keep fighting it the entire time otherwise.
If a client's car has a real odor issue, be upfront that the fragrance step comes after the actual fix, not instead of it. Clients respect that honesty, and it protects the fragrance's reputation with them too, since a client who associates the scent with a smell that came back within days is learning the wrong lesson about a product that was never given a fair shot to begin with.