Preparation is the entire job
The visible part of painting takes a fraction of the time. Everything that determines whether the result lasts happens before any colour goes on.
Filling and sanding so the surface is flat. Degreasing, particularly anywhere near a kitchen or a workshop, because paint will not bond to a film of grease no matter how good it is. Priming where the substrate demands it - new plaster, bare metal, patched areas, or a dramatic colour change. Masking properly so the edges are clean and the floor is not part of the job.
Skip those and the wall looks correct on handover and fails afterwards: peeling at patches, sheen differences where the filler drank the paint, and edges that need cutting in again.
When two quotes for the same room are far apart, this is almost always the difference. It is worth asking what preparation each one includes, because that turns a mysterious price gap into a specific comparison.
Working around a trading building
Most commercial painting is scheduled around when the building is empty rather than when the painters are free.
The practical unit is the section. One area at a time, prepared, painted, dried and returned to use before the next area starts. That keeps the disruption contained and means there is never a morning when a large part of the building is unavailable.
Dry time is the constraint that decides everything else. It is affected by the coating, the temperature and the ventilation, and it is the one thing that cannot be compressed by adding people. A schedule that ignores it produces marked walls and furniture stuck to skirting boards.
Exteriors are a substrate question
Exterior work is decided by what is underneath far more than by which product goes on top.
Render, masonry, timber, metal cladding and previously coated surfaces all need different preparation and different systems. Failed exterior paint is nearly always a bonding failure at the substrate rather than the coating wearing out - which is why the survey matters more than the brochure.
Exposure does the rest. The elevation that takes the weather will always be on a shorter cycle than the sheltered one, and planning around that is cheaper than repainting everything to the schedule of the worst wall.
Keep the specification
The most common avoidable cost in painting is the touch-up that does not match.
Something gets damaged, a repair is made, and a year later nobody knows the product, the colour code or the sheen level that was used. The patch is then either visibly different or the whole wall gets repainted to hide it.
We keep the specification per area and hand it over. It costs nothing at the time and it is the difference between a fifteen-minute touch-up and repainting a room.