I’d like to hide a mesh but hiding polygon piece by piece with the rectangle selection is a pain. can i paint a mask and hide a part of it ?If so, how ?
If you have used zspheres to create your model you can click frame button left of edit button to see different group regions of your model.
Now if you shift+ctrl+click on one particular region others will be hidden.
This will make your life easier.
Well you can try masking but it might prove to be a bit more time consuming then normal rectangle hiding.
Depends on how good you are at masking.
I am not, so I dont prefer it. 
When part of the model is masked, there are Hide and Show options in the Tool>Masking menu. My Select-It ZScript also provides options using this technique and allows you to paint your mask using Projection Master, then automatically change that to mesh visibility.