PCB Coating

I notice that a lot of printed circuit boards have a clear coating on part or all of the board. Can anyone tell me what it Is and is that something that can be bought in an aerosol can?

Search for “conformal coating”.

I can't say exactly what Rockwell uses, but I do know that it's principally to protect against tropical humidity and corrosive atmospheres.

My understanding is that the goal is to meet the "G3" level of the ANSI S71.04 standard for corrosive atmospheres.

In another life when we made circuit boards for the military we used a product called Humiseal:
http://www.humiseal.com/conformal-coating/
It was their spec so I guess it worked.

Yes, conformal coating.
The type I used while doing repair work was mostly acrylic to seal. Unless the board was previously sealed with silicone, so I could match the thickness up to look like factory.
But if you use epoxy or silicone and the board comes back on warranty it is harder to access your components again.

Yes you can buy in spray cans.