I have been using a sort of a very strong hairspray called 3DLAC for about 2 years directly onto the aluminium heat bed of the Anet A8 printer I have.

Basically, all those sprays contain copolymer constituents, PVA (PolyVinyl Alcohol), Vinyl or Acetate. These are also found in certain glue sticks or wood glues. For me this spray works perfectly! On day one I assembled the printer, the paper tape tore and I was too anxious to wait for new tape to arrive. This worked so well that I have not changed it for that printer.

Cleaning is very easy as PVA or any of those constituents are solvable in water, so a moist cloth or paper towel over the plate is all to clean it. Furthermore, you do not require to spray before every print.

To answer your question if you should use a PVA based spray like hairspray directly onto the metal build plate is a matter of preference, but you definitely could use it as I have been doing it for about 2 years.

To address the comments:

I spray the heat bed platform whilst it is attached to the printer. I do pull it forward and gently spray the bed or just the location where the print is going to be build. Note that you do not need to do that for every print. I recently did notice very little spray on the X guide rods (maybe I have been careless once or twice), but that has not been a problem for my Chinesium iGus ripoff plastic bearings. It is very easy to clean with a damp cloth. It also works great on the glass bed of my Ultimaker 3E, but I usually (unless when I'm lazy ;) ) remove the slate of glass before printing. You could consider shielding the rods with a piece of paper, but it has not been necessary for 2 years.