Via Fill is a method to completely close the via holes with a hole filler.
This technique can be applied for holes =<0.6mm.
These days due to the extensive use of SMD components, the via fill technique is no longer popular as it affects the planarity of the board due to the protrusions of filler out of the board surface (up to 50µm).
The alternative for this is to screen print solder mask in the holes first before applying solder mask on the entire board. That is the via fill technique currently most frequently used and used by Eurocircuits to perform Via Fill.
See our design guide lines.