Drain and Acid Wash Standard

Purpose

Emptying a pool is the most destructive thing this trade does on purpose, and the damage is not from the acid. It is from the shell floating out of the ground on groundwater nobody checked for, from a plaster surface that had no thickness left for another wash, and from acid vapor collecting in a deep end nobody ventilated. This procedure guarantees the water table is measured before a gallon leaves, the surface is assessed for remaining thickness at the first pass, the effluent is neutralized to a stated pH before it goes anywhere, and nobody works in an empty deep end without respiratory protection and a person topside. What the customer gets is narrower and more honest: a stain removed at a countable, recorded cost in surface thickness, not an open-ended wash that ends in exposed aggregate.

Scope

Covers full drain and acid wash of a gunite or shotcrete pool with a cementitious interior finish: the hydrostatic gate, discharge, drain, the wash, neutralization, and the handoff to refill.

It does not cover vinyl liner pools, which are not drained as a service operation because the liner shrinks and the wall can deflect without water behind it, nor fiberglass shells, which drain only under the shell manufacturer's written procedure. It does not cover the no-drain acid treatment, and it does not decide whether a wash is the right treatment at all: the stained-surface decision matrix owns that. Refill chemistry and brushing belong to the plaster startup protocol.

Roles and responsibilities

Procedure

1. Prove the water table is below the pool floor before any water leaves. Auger a test hole within about ten feet of the shell, bottoming at least a foot below the pool floor elevation, and watch it for at least an hour. Accept the drain only when the hole stands dry over that hour. Wrong looks like water standing or rising in the hole: stop, do not drain, and route the job to the no-drain method, because an empty shell in saturated ground floats and a floated shell is a total loss, not a repair. Repeat this gate if the job is postponed past a rain event; the reading is only good for the day it was taken.

   deck level
   ==================        ====
   |                |        |  |  test hole, within
   |   pool shell   |        |  |  about 10 ft of shell
   |                |        |  |
   |________________|        |  |
       pool floor            |__|  bottom sits at least
                                   1 ft BELOW pool floor

   any standing water in the hole = do not drain

1. Pull the hydrostatic relief plug from the main drain sump once the water is low, and leave it out. That plug is the shell's only vent against uplift, and a pool can float on a rainstorm mid-wash even when the pre-drain hole was dry. Accept when the plug is in your hand and the sump is open. Wrong looks like a seized plug a tech gives up on: stop the drain at that level, refill enough to hold the shell down, and escalate for a plug extraction before continuing.

2. Confirm the discharge route in writing and set it up before the pump starts. Chlorinated, low-pH water does not go to a storm drain, a creek, or a neighbour's yard. Accept when the route is a sanitary cleanout or another destination the local sewer district or water authority has approved for this address, with the approval recorded on the job. Wrong looks like a hose run to the street because the cleanout is inconvenient; that is a violation in most jurisdictions and the fine follows the shop, not the homeowner. Confirm the receiving system's flow limit and throttle the drain pump to it rather than backing sewage into the house.

3. Rinse the shell of hypochlorite residue as it empties, and let it drain clear before any acid is opened. Acid landing on wet hypochlorite releases chlorine gas at a rate that will drop a person in a deep end in seconds. Accept when the last pool water is out and the surface has been hosed and drained clear. Wrong looks like starting a wash with a green puddle still in the drain pot: flush it out and let the sump run clear before acid comes off the truck.

4. Set up ventilation, protection, and the topside attendant before anyone enters the empty shell. Acid vapor is heavier than air and collects in the deep end, which does not clear itself. Accept when a fan is moving air across the deep end, the person entering is in splash goggles, face shield, chemical-resistant gloves and boots, and an air-purifying respirator with acid gas cartridges under the shop's written respiratory protection program, and a named attendant is on the deck watching them. Wrong looks like one tech alone in a deep end with a face shield and no respirator: a face shield does nothing for an inhalation route. No fit-tested respirator on the truck, no wash today. An empty shell in summer is a reflector oven, so rotate on a schedule and keep drinking water on the deck.

5. Pre-wet the surface, then apply the weakest dilution that moves the stain, in bands, rinsing each band immediately. Wet plaster takes acid evenly; dry plaster blotches and etches. Dilute in a watering can by adding acid to water, never water to acid. Start weak, around one part muriatic acid to ten parts water, and step up toward the one-to-one working mix only where the weak pass demonstrably fails. Accept a band when the stain lifts and the surface still shows uniform cement paste. Wrong looks like acid left to dwell while the tech walks the next band, which burns a hard line you cannot blend out. Work top down, keep a hose running behind you, and never step onto an acid-wet slope.

6. Stop at the first sign of aggregate exposure and do not run a second pass on that area. The wash removes cement paste, and once sand or pebble stands proud there is nothing left to take. Accept an area as finished when the colour is even and the surface is still closed. Wrong looks like a patch reading rough under a gloved hand: that surface is at the end of its washable life, so stop, photograph it, escalate, and change the conversation from stain removal to resurfacing rather than chasing an even colour into the aggregate.

7. Neutralize the effluent in the deep end to a stated pH before it is pumped anywhere. Broadcast soda ash into the standing wash water, agitate, and titrate. Accept a pH between 6.5 and 8.5 on a sample drawn from the standing water, not from a hose. Wrong looks like pumping a deep end still reading in the low fours because the water looked clear; clarity is not pH. Re-dose and re-test until it passes. Broadcast from upwind with goggles on, because the reaction with residual acid throws a mist.

8. Refill immediately on completion and start the plaster startup procedure the same day. Reinstall the hydrostatic relief plug and confirm it seated before the fill reaches the main drain, and record the reinstall next to the removal, because a shell filled against an open sump loses the fill to ground and leaves nobody able to say whether the plug is in. A fresh-washed surface left dry in sun crazes, and a shell left empty overnight has another night of weather to float in. Accept when the fill is running, the hose is weighted at the drain rather than dropped on the wall, and the brushing schedule has a first date. Wrong looks like leaving a washed pool empty over a weekend because refill was billed separately. If fill water is not available that day, the wash does not start.

How many washes a surface has left

A cementitious interior finish is applied as a coat of finite thickness, commonly three eighths to half an inch, and every acid wash removes some of the cement paste at its face. The number of washes a surface can take is therefore finite and countable, and smaller than most customers assume. A shop that does not track wash count eventually sells a wash to a surface with none left, exposes aggregate, and owns the argument about whether the acid caused it.

So the count lives in the property record, not a tech's memory, and it is disclosed before the job is sold. Where the history is unknown, treat the first pass as the assessment: weakest dilution on a small area, look at what it does, and price from what you see rather than from the phone quote.

The record this produces

The drain and wash record, attached to the property rather than only to the job, because its whole value is cumulative. Fields: test hole location, depth relative to pool floor, observation times, standing-water result; hydrostatic plug removed and its condition; discharge destination and approval reference; respirator and cartridge type plus attendant's name; dilution per area and whether it was stepped up; areas washed with the pass count on each; the aggregate-exposure assessment with photographs; effluent pH before pumping; refill start and first brushing date.

The service manager reads the pass count before quoting the next wash here. The next crew, possibly years later, reads the cumulative count to know whether this surface can take another one at all. The office keeps the discharge approval reference, because that is what the district asks for.

Worked pass: a stained plaster pool with an unknown wash history

Property record shows a resurface roughly nine years ago and no wash count since, so the history is unknown and the first pass doubles as the assessment.

Step 1: test hole augered about eight feet from the shallow-end wall, bottomed roughly two feet below pool floor elevation, watched 8:20 to 9:25. Dry throughout, so the gate passes and the drain starts.

Steps 2 through 4: hydrostatic plug out at about knee-depth of remaining water and left out. Discharge to the sanitary cleanout at the side of the house, throttled so it does not surcharge, district approval reference on the ticket. Shell hosed as it drained, sump clear before acid leaves the truck.

Step 5: fan at the deep end, lead tech in goggles, face shield, gloves, boots and a fit-tested half-mask with acid gas cartridges. Attendant named on the ticket. Ambient in the low nineties, so rotation set at 20 minute working intervals.

Step 6: shallow-end wall pre-wet and washed at one part acid to ten parts water. Stain lifts, surface stays closed and even. Same dilution carried down the slope in bands, rinsing behind.

Step 7 FAILS on the deep-end wall. The weak pass lifts the stain unevenly, and where it lifts the surface reads gritty under a gloved hand with grains standing proud. Under the stop rule no second pass is run and the dilution is NOT stepped up. The area is photographed, deep-end work stops, and the service manager is called from the deck. The customer conversation that afternoon is not about finishing: the shallow half cleaned up, the deep-end wall has reached the end of its washable thickness, and the honest options are an uneven deep end or resurfacing. The shop does not get to make the deep end match.

Step 8: standing wash water reads pH 4.2 on the first titration, failing the 6.5 to 8.5 gate. Soda ash broadcast and agitated, retested at 5.9, still failing, dosed again and retested at 7.1. Passed, then pumped.

Step 9: fill running by mid-afternoon, hose weighted at the main drain, first brushing set for the following morning. The property record now shows one wash on the shallow half and one aborted pass on the deep end with photographs attached, which is what the next crew needs.

References

• Local sewer district or water authority rules for pool water discharge; chlorinated, low-pH water to a storm drain or surface water is prohibited in most jurisdictions and approval is site-specific.
• 29 CFR 1910.134 for the respiratory protection program a fit-tested acid-gas respirator must run under, including written program, medical evaluation and fit testing.
• The muriatic acid safety data sheet from the supplier of the product on the truck, for exposure limits, incompatibilities, and first aid at that concentration.
• See related: the PoolService stained-surface decision matrix for choosing the treatment, and the plaster startup brushing protocol for the refill.