Preparation
Surface preparation: profile, method and hours
Preparation decides what the coating has to grip, and once the floor is finished it is hidden under the coating, where it is hard to check. This page derives the profile your film thickness needs in this site's planning bands, works out which method can reach it on your slab, and adds up the machine and hand hours.
Preparation method for this slab
Diamond grind
to reach profile 4, in 2 passes
Profile note Profile 4 in this site's planning bands: a thicker film gets a coarser texture to key into.
Why one method and not another
These four are not interchangeable and they are not ranked by price. In this site's model each reaches a different depth of profile, and two of them cannot remove what is already on a slab. The profile each method reaches is one of our own planning bands, listed with the other allowances on the methodology page.
| Method | Profile reached | Removes coatings | On this slab | Why |
|---|---|---|---|---|
| Acid etch | up to profile 2 | no | ruled out | Ruled out: acid does not touch concrete carrying a cure-and-seal or a penetrating sealer. It reacts with the cement paste, and here the paste is covered. |
| Diamond grind | up to profile 4 | yes | usable here | In this model diamond grinding removes what is on the slab and reaches up to profile 4. |
| Shot blast | up to profile 5 | no | usable here | In this model shot blasting reaches up to profile 5 and is chosen for 900 square feet or more of open floor at profile 4 or deeper. It does not work the edge band, which is hand-ground. |
| Scarify | up to profile 9 | yes | ruled out | Ruled out in this model: scarifying is kept for profile 6 and deeper, and this build needs less. |
The acid-etch question, answered properly
Acid reacts with the exposed cement paste at the surface to leave a light texture. In this site's model that reaches about profile 2, which suits a thin sealer on bare concrete but not a build coat, and acid does nothing where a sealer, coating or oil covers the paste.
Etching also leaves residue that has to be neutralised, rinsed and dried, and a slab that has just been wetted is a poor surface for a moisture-sensitive resin until it dries again. Count that drying time when comparing etching with grinding.
This site rules etching out whenever the derived profile is 3 or deeper, or whenever anything is sealing the surface. At the settings above, the reason it gives is: Ruled out: acid does not touch concrete carrying a cure-and-seal or a penetrating sealer. It reacts with the cement paste, and here the paste is covered.
Where the hours come from
Machine hours are the machine-reachable area, multiplied by the number of passes, divided by a productivity rate. Hand hours are the strip around each wall and obstruction that no walk-behind machine can enter.
| Quantity | Value here | Source |
|---|---|---|
| Gross floor area | 384 sq ft | Your own measurement |
| Perimeter plus obstruction edge | 80 ft of wall, plus obstructions | Derived from your dimensions and obstruction count |
| Edge band no machine reaches | 4 in | Assumed — machine housings vary |
| Grinding productivity | 180 sq ft per machine-hour, one pass | Assumed |
| Shot-blast productivity | 500 sq ft per machine-hour, one pass | Assumed |
| Hand edging productivity | 45 sq ft per hour | Assumed |
| Edge added per obstruction | 8 ft | Assumed allowance |
| Clean-down between passes | 1 h per 1,000 sq ft per pass | Assumed allowance |
| Getting the machine in and out (step, narrow door or ramp) | 1.5 h | Assumed allowance |
| Acid-etch coverage | 400 sq ft per hour | Assumed allowance |
The pass count is derived from what is on the slab: a sealer adds one removal pass in this model.
The total is those four components added: 3.9 machine hours, 1.4 hand-edge hours, 0.8 hours of clean-down between passes, and 1.5 hours getting the machine in and out. That comes to 7.6 hours, and it is written out because a total whose parts are not all on the page is a total nobody can check.
Things about this slab worth knowing before the machine arrives
- An hour and a half (our allowance) has been added for getting the machine in and out past the step, narrow door or ramp.
Biggest swing factor The pass count. Each extra pass repeats the machine hours, the hand-edge hours and the clean-down for the whole floor.
What to do with this
- Grinding concrete raises silica dust. The dust control page reads the federal silica table for floor and handheld grinders, and says so where the table has no row (it has none for shot blasting).
- Cracks and spalls are repaired after preparation and before coating, on the repair takeoff.
- These hours feed the labour lines on the installed cost page at whatever rate you are actually being charged.