Get yourself one of these pressure pots.
Coffee Stirrers
Regards,
Tom
When I go to starbucks I usually pick up a dozen or so coffee stirrers. In addition to spreading glue, I like using them for mixing and applying 2 part epoxy.
For box joints I use a foam brush cut into the same size fingers. This allows one swipe to cover all the mating surfaces. On flat surfaces a credit card works well, and is easily trimmed to the width you need for mortises.
I like the silicone brushes from rockler. They have a couple different sizes depending on how big you mortise is.