If the manufacturer of the epoxy recommends you etch your concrete
first, then I'd do it. Otherwise, a poured concrete slab is a rough and
porous material that any coating should stick well too without having to
make that surface rougher still.
But, to answer your question, once you etch that concrete, you make it
rougher, and it stays rougher until you do something else (like polish
it) to make it smoother. It may get dirtier if you leave it for a while
before painting it with epoxy, but it's not gonna get smoother on it's
Fresh concrete is highly alkaline, and so oil based paints (and
inexpensive latex paints) won't last on fresh concrete. You can use a
concrete primer which are made specifically for painting over fresh
concrete, but these are essentially latex primers, and it's never a good
idea to put a hard strong coating (like epoxy) over a softer weaker one
(like latex primer) cuz then the harder coating will be prone to
"chipping" because of the weaker coating under it breaking when it
receives an impact or can't resist some force applied to it.
Typically it takes at least a year for the alkalinity in fresh concrete
to subside, so the general rule is not to paint concrete for the first
two years of it's life. However, I don't know how resistant epoxy
coatings are to alkalinity, so that rule might not apply to epoxies.
What you SHOULD do is buy a small roll of vapour barrier and tape pieces
of that plastic together to cover your garage floor. Tape the perimeter
of the plastic down, and slip a hygrometer (measures relative humidity)
dial side up under the plastic before you completely finish taping down
the perimeter. Lee Valley and most hobby shops will sell hygrometers.
Watch that hygrometer needle. If the concrete is more than a few weeks
old then any water evaporation from it won't be the mixing water any
more. It'll be moisture migrating up through the concrete slab from the
ground (or crushed limestone base) below. And, if moisture is coming up
through the slab, I'd be reluctant to paint that floor with epoxy unless
and until I measured how much moisture was evaporating and whether that
fell within the guidelines of the company that makes the epoxy floor
paint you want to use. That's because if you have too much moisture
evaporating from the concrete it can build up sufficient pressure to
push any coating off the concrete, even epoxy. In that case you'll find
that your epoxy floor is peeling in places.
By covering the floor with plastic, you trap any escaping moisture, and
it becomes a steadily increasing relative humidity under the plastic.
If you see the hygrometer needle rising, then expect to see condensation
first forming on the underside of the plastic over the area(s) where the
moisture is evaporating from.
When I replaced the Olefin carpets in the living rooms of my apartment
block, I'd put the old carpets in one of my sister's garages. That gave
them a flooring that wasn't slippery when it was wet, absorbed oil and
grease to keep the underlying concrete cleaner, was easier on the knees
to kneel on and warmer to lay on when doing any repairs to their cars.
And of course, it cost them nothing and those old carpets weren't any
good for anything else.
If you find that there's moisture coming up through your concrete slab,
phone around to the carpet retailers in your area and ask to speak to
the installation managers. Ask the installation managers who sells
flooring installation supplies in your area. Those installation supply
stores should carry "calcium chloride test kits" (IIRC) which are used
to measure the amount of moisture coming up through the concrete floor.
Every flooring manufacturer will have guidelines as to how much moisture
his flooring will allow to evaporate through it. You might want to get
one of those kits and see if the moisture evaporation rate from your
slab is within the guidelines of the company that makes the epoxy floor
paint. Set up your test kit in any areas where you see condensation
forming on the underside of the plastic over your garage floor.
Park your car(s) outside while you're testing the slab for moisture
Or, at least, that's the way I'd proceed with this.