On Fri, 10 Jul 2015 20:24:56 +0000 (UTC), Cydrome Leader
wrote:

I need to drill about 100 holes (around 5/16") in 1/8" hot roll plate. I
usually just use cutting oil from the hardware store, works fine, makes a
huge mess. Steel tube will be welded to the plates, each drilled with with
4 holes for fastening to supports for countertops.

Welding instructor uses a spray bottle of a water based coolant, something
milky looking. It's not as messy and doesn't make everything rust either.
I tried it, but with a dull bit, it wasn't really a test of anything
meaningful for the short time I tried it out.

Any real difference between the two for hand drilling or use with a drill
press, or good reasons to pick one over the other for specific jobs?
I make my living as a machinist using CNC and manual machines.
Virtually all the machining I do uses water soluble oil at about 8%
concentration in water. It works great. A big enemy of HSS drills is
overheating. The drill steel gets relatively soft and dulls. When hand
drilling this can be a problem because it is often hard to apply
enough pressure to keep the drill bit cutting and the operator often
has the drill spinning too fast. So a water soluble oil coolant works
well because not only does it supply high pressure additives it also
removes heat very fast and keeps the drill bit cool.
Eric