If you have a plaster pool, you might want to read this

Plaster pools have special needs as far as water chemistry is concerned and are far less forgiving when the water is out of balance.  Below is an excellent read.  It is a direct copy and paste from http://www.poolcenter.com/plaster that we thought would provide valuable information to our readers.

Thank you,
Robert Flynn
Pool Water Experts

                                                                         
                                                                                  Plaster Pools

Concrete pools are the oldest form of building pools. The process starts with digging a hole in the ground and lining the hole with rebar. Then the drains, skimmers, return lines, and in-floor cleaning system are set into place. After that, the pool is shot with gunite. Then, the tile and coping stones are set in place.  Finally, the pool is shot and troweled with plaster. Once the plaster is finished, the company usually starts filling the pool with water.

Once the pool is full, the system is started up and balanced with chemicals. This is the time that takes the most work, usually a lot of brushing and backwashing due to dusting from the new plaster.

Also called white coat or marcite, plaster is an age old process of finishing many structures. Used underwater, it provides the watertight seal that the more porous gunite or shotcrete beneath it cannot. Plaster finishes provide twenty years of service under ideal conditions, however, ours is rarely an ideal world. The plaster surface is meant to degrade slowly, eventually requiring a fresh coat.

Plaster has shading variations?
Known as mottling, and distinct from surface stains and deposits, mottling may appear as grayish hued areas. Rarely do I see a plaster job that is white like milk. They're usually white like clouds. Being a natural product, mottling is inherent in any plaster job and should not be considered a defect. There has been much research on the subject in recent years, however no consensus has been reached on the causes and cure of mottling.

Plaster is stained?
Different from mottling, stains are usually localized and distinct in color from the plaster. Most stains are caused by heavy metal minerals coming out of solutions and depositing themselves on the nearest structure they find. They may also remain suspended in water, providing dramatic new color schemes. Below is a chart listing common metals found in pools, where they came from, and their identifying colors.

Plaster has crystalline deposits?
Your pool water has precipitated metallic salts out of solution. This will create areas of the pool which are extremely rough and scaly. There are several solutions to remove these deposits, including acid washing, sanding and the use of a calcium deposit dissolving chemical called HydroQuest.
If any of the above causes apply to you, your pool should be using a sequestering agent which aids your water balance in preventing the metal from dropping out of solution and staining underwater surfaces. Most stains left by precipitated metals can be removed by one form or another. An acid wash every few years will keep a pool looking nice.  

Want to increase the life of your plaster?
Barring problems in the mixing, application and curing processes, the pool owner, or service company controls its condition and life span. Keeping your water chemistry in balance and most importantly, preventing corrosive water environments of low pH and low alkalinity will reduce wear and tear.
 Maintain your pH levels at between 7.4 - 7.8.Alkalinity should be kept between 80 - 120 ppm. Lower levels may etch your plaster, which provides a safe harbor for dirt andalgae to grow, isn't attractive and is rough to the touch.

Heavy metals cause stains.
Metallic salts cause scale. These salts are primarily forms of calcium and magnesium which can deposit on your plaster, pipes and equipment. They may arise from the use of calcium based sanitizers or the fill water may have high calcium hardness levels.
Keep your calcium hardness levels between 200 - 400 ppm.
​A level that is above this may find it easy to precipitate out of solution. This is known as a scaling condition. Conversely, water with low levels of hardness will produce an aggressive condition. In aggressive conditions (soft water), the water will take the calcium it wants directly out of your plaster, resulting in plaster breakdown and bond failure.
Pool plaster is pitted or etched?
If your plaster has surface irregularities, which may take on a beige hue, you have what's commonly called etching. This etching can be caused by low pH or alkalinity; an acidic condition. It may begin within the plaster, from the original mix on application, or etching may start from the gunite side of the plaster and work itself from the outside in.
Pool plaster has large, dark areas?
You may be seeing the gunite or shotcrete beneath the plaster beginning to show through. You better start budgeting for that re-plaster.
Pool plaster has small cracks?
Known as crazing or checking, the tiny, barely visible cracks are usually caused by extreme temperature variations, especially during initial curing. These are not actually cracks, and pose no structural hazard or danger of leaking. Acid washing could remove the crazed layer. Larger cracks should be cut out in a butterfly or dovetail fashion with a 4" or 7" grinder, and filled with a plaster patch mix or a flexible sealant can be used where further movement is suspected.

Plaster has delaminated?
Known as bond failure, this will occur as areas where the plaster has popped off. Usually seen on re-plaster jobs, where the plaster to plaster bond may never be as strong as the original plaster to concrete bond. This is repaired with a plaster patch. Plaster normally does not delaminate from the gunite;  this bond failure can more easily occur, plaster to plaster. Chip away any loose material around the edge of the delaminated area. This will sometimes make the patch much bigger than the hole is to begin with. Chip & chisel to break up and rough up the surface, clean and brush on a bonding additive. Then pour in premixed white Portland cement and marble dust; 2:1. Use bonding additive (like Acryl 60) to mix the plaster with. Trowel smooth with a round ended trowel. Keep the patch from drying too quickly by doing this repair in the evening and covering with moist towels or burlap.

Plaster patching: The delaminated area is chipped outwards until good bond is found. The surface is roughened and plastered istroweled in place by hand. Cost depends on the size and location of the patch. Plaster patching material can be purchased at most pool stores. It is a mixture of white Portland cement and marble dust. A bonding additive such as Acryl 60 should be used when applying.

Re-plastering: The pool is drained and acid etched several times. Chipping and cutting around all fittings and under the tile allows the new coat to be feathered. A scratch coat is applied first, followed by a finish coat. The pool is filled immediately; the plaster cures underwater. The surface must then be brushed twice daily, with vacuuming and continuous filtering to remove the plaster dust. Cost for a re-plaster is based on surface square footage. Expect $2,500 - $4,000.
Once upon a time, plasterers would plaster a pool in any color, as long as it was white. Black or gray plaster is now becoming more common, and other colored additives can produce varied color effects. Any colored plaster will show more mottling than white pools, however. If you plaster in any color other than white, expect streaking and uneven hues. Don't expect flat black color, for example.

Pool painting: Before plastering became popular, pool shells were painted. These were low grade paints that barely lasted one season. The annual drain & paint was gladly given up when plaster became popular. The underwater epoxies used today offer a 7 year life span for about half the cost of re-plastering. Paint adheres very well to properly prepped plaster, and offers a wide variety of colors and graphic capabilities. Pool painting is a second class choice to re-plastering. It may be half the cost, but will last only half as long (at best). If you start painting, you keep painting, unless you sandblast it off to allow re-plastering. We tend to discourage our customers from painting a pool. A few dollars saved in the short run will cost more in the long run.

Pool painting: The pool is drained and degreased. It is then acid etched and degreased again. After drying, a primer coat is applied, followed by two coats of the chosen color(s). Again with the drying, and the pool is filled. Price will vary according to size and prep factors. Expect $1,900 - $2,800. Pool artwork varies.

Want to Re-plaster the pool yourself?
I would discourage this. Plastering is nothing short of an art and science. Experienced plasterers know how to get the mix right, with numerous variables that can affect the finished product. They also work in a team of 4 - 6 workers, to get it up before it sets (and cracks). Small patches? OK, but re-plastering the entire pool? I would leave that to plasterers. If YOU prep the pool for plaster, however, you could save 25% or so on the job.
​
What is the procedure for starting up a freshly plastered pool?
There are basically two methods, chlorine start, or acid start. I do the acid start which eliminates the brushing and filtering of plaster dust. The idea is to drop the Total Alkalinity level to zero, and then rebuild it with the carbonates that are contained in the plaster dust.
Here's how I do it. As the pool is filling, immediately after plastering, add 1 gallon of muriatic acid directly to the water (don't splash it on the bare plaster) per 5,000 gallons of fill water. With a watch, time the amount of time it takes to fill up a 1 gallon or 5 gallon bucket with the hose(s) used to fill up the pool. Do the math to determine how many gallons are added every hour. Divide 5,000 by the number of gallons coming out of your hose(s) per hour, and you will know how long it takes to fill 5,000 gallons from the hoses.
When the pool is full, test the Total Alkalinity. It should be zero. Then test the pH, and do a base demand test to determine how much pH increaser to add. Add up to 6 lbs of pH increaser at a time, brushing the pool to distribute fully. Wait a few hours in between 6 lb additions. When all of the pH increaser is added, recheck pH and Total Alkalinity. Add additional increasers if needed.
Check Calcium Hardness levels. If below 180 ppm, add Calcium Chloride in dissolved form to bring the levels up. Brush pool after any addition of chemicals. When pH, Alkalinity and Calcium levels are balanced, slowly begin to chlorinate the pool. Do not shock the pool for a week or so. Leave the pool filter (and heater, and pool cleaner) off until the chemistry is balanced. Brush frequently to distribute chemicals added.