How big a hole?

The size of vent and drain holes depends on  the volume of the section being drained. The larger the volume the larger the hole area is required. This can be by a single large diameter hole, or by several smaller diameter holes. Remember that it's not water that is being drained out, it's molten zinc, the consistency of liquid honey. The larger the hole, generally the better the surface finish will be. If it's not necessary to close the end of a hollow section, consider leaving it totally open for the best result of all. Where a hollow section is long, the hole size needs to be increased. The table below is for 2m sections. Double these holes sizes (or number) for double the length, etc.

Diameter of section        Minimum Hole size mm

< 25mm                                        10

25-50mm                                      12

50-100mm                                     18

100-150mm                                   20

>150                                              25