The trouble with most of the expensive ones people are recommending are simple, for modelling purposes they truly are overkill, unless you are spending extended periods in a closed room with epoxy/polyester resins or volatile paints.
For the most part you will be worried about dust particulate from sanding etc, for that a simple cotton filtered mask is more than adequate. As I mentioned above, I use this type, as you will see, the don't sit on the face, but hook under the chin making them much more comfortable to wear.
( edit: 3M 8822 Dust/Mist Respiratory Valved Face Mask )