I use an SP Systems backpack sprayer - I like it because it has a jet
agitation system that keeps swirling the liquid while I work and reduces
the need to "jump around" to keep the powders from settling. I have
used this one now for three or four years. 4 - 5 gallons is all I need
usually.
If I get into a big spray job I have an 18 gallon tractor or
mini-trailer mountable power sprayer, but it doesn't agitate, so I have
to move fast with that or stop and stir!