Grills of doors and windows are always painted before they are used

i.                     Usually grills of doors and windows are made of iron.

ii.                    Iron objects get rusted (Fe₂O₃. H₂O) on exposure to moist air.  

iii.                  Since air always contains moisture, iron grills get rusted.

iv.                  Painting iron objects prevents rusting, hence iron grills of doors and windows are always painted before they are used.