Preparation is most of the job

Coatings rarely fail because the coating was bad. They fail because the surface underneath wasn't ready: it was too smooth, too dirty, sealed, weak on top, or giving off moisture. Preparation is slow, loud and invisible once the floor is finished, which is exactly why it's the first thing a low bid cuts. If you only ask one set of questions, ask about prep.

Why acid etching isn't enough

Acid etching, scrubbing a diluted acid into the concrete and rinsing it off, is the method many DIY kits rely on. It produces only the smoothest profile on the industry's scale (the ICRI chart associates acid etching with CSP 1). It won't remove sealers, curing compounds, paint or oil, which acid doesn't dissolve, and it doesn't reliably open a hard, tightly troweled surface. It also floods the slab with water that has to dry out before coating, the opposite of what you want on a floor that's about to be sealed. Professional systems call for mechanical preparation instead.

Grinding and shot-blasting

Diamond grinding. Machines with diamond tooling grind away the top layer of concrete along with sealers and contamination, leaving a clean, open surface. Good contractors connect grinders to dust extraction with HEPA filtration; ask about dust control, especially indoors.
Shot-blasting. A machine throws steel shot at the floor and vacuums it back up, leaving a rougher profile. It's common on large commercial floors.

Concrete surface profile (CSP)

The International Concrete Repair Institute publishes a set of ten replica chips, CSP 1 (nearly flat) to CSP 10 (very rough), used to describe and check how rough a prepared surface is (DeFelsko/ICRI). The coating's manufacturer says what profile its product needs; thin coatings call for a smoother profile than thick overlays. Ask the specialist which CSP the product requires and how they'll check that the floor reaches it.

Moisture testing

Two ASTM methods are used on concrete floors:

ASTM F1869, the calcium chloride test, measures the moisture vapor emission rate at the slab surface, in pounds of moisture over 1,000 square feet in 24 hours.
ASTM F2170 measures relative humidity inside the slab using probes placed in drilled holes.

Results describe the slab only at the time and the spots tested, so tests should be spread across the floor. Each coating's data sheet says what moisture level it tolerates. If the slab tests above it, the usual answer is a moisture-mitigating primer rated for that level, not hoping for the best.

Cracks, spalls and joints

Hairline shrinkage cracks, wider structural cracks, spalled (flaking) areas and control joints all need decisions before coating. Cracks are typically cut open with a crack-chasing blade, cleaned, filled with an epoxy or polyurea filler and ground flush. Control joints may be filled or honored (left as joints), depending on the slab. Cracks that are still moving will usually reappear through any coating; ask the specialist what they can and can't promise about cracks.

Temperature, humidity and the dew point

Prep is also about conditions on the day. Coatings have minimum and maximum temperatures and humidity limits, and condensation on a slab can ruin adhesion. One floor epoxy data sheet, for example, requires the substrate to be at least 5°F above the measured dew point (Sika). Ask how the crew measures and records conditions during the job.

Questions to ask

Will you grind or shot-blast, and what profile does the product require?
How will you control dust?
Which moisture test will you run, how many, and where?
What will you do if the moisture results are high?
How will you repair cracks, spalls and joints?