Brown and rust stains on the plaster: metal, or organic?

Stains on plaster fall into two families, and telling them apart is the whole job. The treatments are different and neither one works on the other, which is why people spend a season trying products that were never going to help.

The test, before anything else

Vitamin C test. Crush a plain ascorbic acid tablet, hold it against the stain for about thirty seconds, and look underneath.

The stain lightens → metal stain, usually iron, sometimes copper or manganese. Ascorbic acid is a reducing agent and it lifts metal.
Nothing happens → try a chlorine tablet held against the stain instead. If that lightens it, the stain is organic: leaves, algae residue, or something that sat on the plaster.

Do this before buying anything. It takes a minute and it is the difference between a $15 fix and a wasted season.

Metal stains

Colours: rust brown and reddish (iron), blue-green to teal (copper), purple-black or brown-black (manganese).

Where the metal comes from: fill or top-up water, particularly well water; corroding metal components in the plumbing or an old heater; an algaecide with copper in it, used repeatedly; and the classic case, something metal left lying on the step.

Treatment: we lift the stain off the plaster with ascorbic acid, then add a sequestering agent that keeps the metal dissolved in the water instead of letting it settle straight back onto the surface. That second step is the one people skip, and it is why the stain comes straight back. If the source is the fill water, this becomes a maintenance routine rather than a one-off.

Organic stains

Colours: brown, reddish-brown, or greenish, usually diffuse rather than in defined spots, and often in the shape of whatever caused them.

Causes: leaves and organic debris left sitting, especially after a monsoon storm blows a tree’s worth of material into the pool; algae that was allowed to root before being cleared; and anything that sat on the plaster long enough.

Treatment: chlorine, held high, plus brushing. Organic staining generally responds if the pool chemistry is right.

When the answer really is an acid wash

Targeted treatment is right for isolated stains. An acid wash is right when:

The staining is widespread rather than in spots.
It has been there through multiple seasons and has driven into the plaster.
The pool was green for a long period and the staining is the shadow left behind.
You have already treated it properly and it did not lift.

The acid wash works by removing the stained layer of plaster, which is also its limitation. A pool has a finite number of acid washes in it, commonly around five, before the plaster is too thin and the answer is resurfacing. So it is worth spending the cheap options first, and worth mentioning if the pool has been washed before.

What it costs

Situation	What it needs	Price
A few metal spots	Ascorbic acid plus sequestrant	Your chemicals
Organic staining, chemistry sound	Chlorine and brushing	Your chemicals
Widespread staining into the plaster	Drain and acid wash	$300 to $800
Oversized or heavily stained	Large acid wash	$700 to $1,200+
Plaster failing as well as stained	Resurfacing prep	Quoted

From the pricing page; the cost calculator prices your pool.

Get a quote

Send a deck photo and a close-up, note the result of the vitamin C test, whether the pool is filled or topped from a well, and whether it has been acid washed before. We cover Mesa, Gilbert, Chandler, Tempe, Apache Junction, Queen Creek, and Gold Canyon.

Frequently Asked Questions

How do I tell a metal stain from an organic one?

Hold a crushed vitamin C tablet against the stain for about thirty seconds. If the stain lightens noticeably underneath, it is metal, usually iron. If nothing happens, try a chlorine tablet held against it instead; if that lightens it, the stain is organic. This costs almost nothing and decides the entire treatment.

Where does the iron come from?

Well water, corroding metal in the plumbing or a heater, fill water, and sometimes from an iron object left on the plaster, which is the classic small round rust spot. A pool filled or topped from a well is the common source in the outlying areas covered here.

Will an acid wash remove them?

Usually yes, because it removes the stained layer of plaster rather than treating the stain. That is the right answer when the staining is widespread. For isolated stains, a targeted stain treatment is much cheaper and does not spend one of the pool's limited acid washes.

What does it cost?

A full drain and acid wash runs $300 to $800 for a typical Mesa pool. Targeted stain treatment on a few spots is chemical work you can often do yourself for the cost of the product, and a fair pro will tell you that rather than sell you a wash.