a poly float is not used for applying plaster.
it is used on render coats, plaster backing coats and floors, to aid the flattening and napping of those surfaces. some material is applied with this float but only in small quantities, where a 'hollow' might need filling.
it is a 'rubbing up' device.
we also stick a nail through one corner in order to scratch a surface, providing a key for the set or skim coat.