You're holding the tire valve against the can with your hand, there's no danger of over pressure, and there's no ignition sources around if the contents happen to be flammable.
We used to do unconscionable things with potassium perchlorate, sulfur, and aluminum powder when you could buy all that stuff at the local pharmacy and no one said boo to you.
It was actually pretty safe to melt saltpeter and sugar and "cast" the mixture into a rocket casing. Not so safe if you didn't design the nozzle right and the thing exploded.
A friend and I used to experiment with "shaped charges" using our supercharged flash powder. My friend made the mistake of selling a couple of them to a school buddy, who dropped one into a covered metal trash can in the hallway in his school and ran off giggling - it shredded the trash can and the concussion broke the windows on that hall. The perpetrator dimed out my buddy as the source of the device, but my buddy never said anything about me, thank goodness, and I was sweating too ...