De or het handwassing?

The correct Dutch article is:

de handwassing

Rule Words ending in -ing: almost always de. See the rule

How to use handwassing

the handwassingde handwassing
this handwassingdeze handwassing
that handwassingdie handwassing
our handwassingonze handwassing
your handwassingjouw handwassing
every handwassingelke handwassing
which handwassing?welke handwassing?
a big handwassing / the big handwassingeen grote handwassing, de grote handwassing