YouTube: https://www.youtube.com/watch?v=gJJjh6upztY
Mixing tin plating solution requires heat, agitation and an absence of air. Manually this is a laborious task and wasteful, because air in the container allows the agitation though it degrades the solution.

With 12V applied to the coil, at room temperature the liquid stays around 50 degrees Celcius. The servo rotates over 90 degrees and has four speeds. The circular plate has lots of symmetrical holes for accommodating different bottles.

The heating coil is smeared with thermal transfer compound and sandwiched between layers of foil. Insulation and tape cover the foil. The coil resistance is 95 Ohms.

 

video demo