This. I decided at a young age that the answer to "can I search" is "no". If I end up in custody as a result, so be it. If they get a warrant and slap me with everything they can, so be it. I cannot take the chance that they will either plant something or find something that I use regularly that might be used illegally and leave me fighting about it for the next few years.

I once built model rocket engines with my son (see sugar rockets online). I kept legal amounts of the mixed chemical as well as the individual components for that in my trunk at various times (going to a launch site, coming home from the hardware store, etc).

In a case like that it's not the officers job to decide if you did anything wrong or not. It's their job to report that you had "bomb making supplies" or some "white powder substance" in your trunk and let the court system sort it out.