When I was little, I used to spend my summers in a farm helping planting and managing the field. We would frequently need to apply liquid pesticide (copper sulfate) on some crops which would require using a face mask and proper protective clothes to not inhale the solution droplets in the air while spraying. I thought this process could be automated therefore I decided to made this small prototype.
Although the robot looks a bit simple, its electronic parts were like a chaos inside its frame. Then I reorganized all the inner parts and simplified the configurations. I designed and 3D printed the blue parts. There is a 12V pump inside which is powered by a 3S1P LiPo battery pack. I used 2 ultrasonic sensors: The one at its side detects any crop, the one at the front detects obstacles. I bought and modified a paint spray nozzle, and attached it to the end of the pump pipe. I programmed this robot in such a way that it revolves each of its wheels exactly for the specified amount to make sure it can drive straight.
I 3D printed the liquid tank in 3 days, in fact it was not waterproof. So I poured some liquid adhesive inside the tank, and shook it for a minute. When the adhesive dried, all the holes inside the tank were filled with adhesive, and it was completely waterproof.