Here is an ex K-9 trainer's advice (if you want to read the whole synopsis - it is here:
https://www.rollitup.org/general-marijuana-growing/32388-never-get-busted-dvd-synopsis.html )
Canines:
The right dog for narcotic detector canine is one that has a strong "ball drive" which is a dog that is naturally "psycho" about finding their ball (or other toy). The officer can scent the toy or ball with a drug and the dog will make the connection between the scent of the drug and his ball.
Barry explains the difference between human and canine noses using "stew" as an analogy. We can see the carrots, corn, peppers, etc.. in the stew but can only smell the combination: the stew. Canines can "see" the different ingredients in the stew with their noses. They smell the carrots, corn, peppers, etc... He says all this to explain that you can not "cover up" or mask the smell of drugs since canines differentiate between the odors. If you cover up the scent with a petroleum product for example, the dog smells gasoline and marijuana - not just gasoline. Masking odors does not work! This includes coffee grounds, mustard, vanilla extract, pepper, fabric softener do not mask the odor of drugs.
Also note that using a strong substance like cayenne pepper or gasoline will cause the narcotic dog to jerk back (because of the powerful scent) which will alert the narcotics officer to something being amiss.
Next Barry discusses how odor permeates its container. So hiding drugs in a gas tank will not work cause over time the odor will permeate the gas tank and a dog will detect the smell. The example he uses is sardines in a zip lock bag. He takes sardines from a can and places them in a zip lock bag at which point no sardine odor is coming from the bag. But after a couple of hours, the odor is detectable because every container is porous to one degree or another and the odor will be detectable by a trained canine. So the dog can't smell "through" anything but the odor does permeate out.
The rate of permeation is different so if you do not contaminate the outside of a container and place a drug inside the container a dog will not alert right away. But over time the odor will permeate anything. So if you are planning on carrying a few joints somewhere get a non-contaminated container (handle with latex gloves) and drop the joints in it and seal it up, a dog should not be able to detect anything for an hour or so.
Contamination should be discussed since touching an illegal substance will contaminate whatever you touch afterwards like the car door handle (and anything else you touch!) which will cause the canine to alert.
Good ideas for confusing a drug detection canine are: hiding in food (the dog handler may think the dog is just excited about food), an animal in the vehicle being searched (or just the scent of an animal), hunter's scent lures and odors (spray the tires and all around the vehicle), and even nearby roadkill can confuse a drug dog.
False alerting is a real problem in that the handling officer can use their voices (and physical cues) to cause their canines to false alert. and they can then search the vehicle. So even if you do everything right, you can still get screwed.