Studies have shown that Coronavirus COVID-19 is predominantly spread via droplets. In that respect a facemask or any face covering such as a scarf, bandana or t-shirt can work, as is currently recommended by the CDC to health care workers if they run out of PPE (personal protective equipment).
While any barrier should be able to stop droplets, those who advise against wearing face masks often say it may lead to a false sense of security (which I believe has never been scientifically shown and was never mentioned in literature with reference to viral spread due to droplets). Theoretically this false sense of protection would be due to an inability to filter out small particles. But has it ever been studied? In fact, it has. While a T-shirt has not done a great job compared to an N-95 respirator, a Hanes sweatshirt did show that it helped somewhat. It filtered out up to 60% of really tiny particles.
This doesn’t mean covering your face with a DIY face mask wouldn’t help to stop the spread of coronavirus. It probably would since Coronavirus is generally spread with large droplets that can be stopped by any barrier. Of course, if you are in an ICU where a patient is getting intubated and airborne particles are generated, an N-95 respirator would be a better choice.
The Hanes T-shirt may not be a Coronavirus slayer but it can definitely provide protection to the most common route of spread, through large droplets. It may even stop some Coronavirus that is spread through airborne particles.