by Mark Garcia March 27, 2019
Removing rust stains from pool surfaces can be difficult - unless you know a few things about the stain, and your pool surface.
Plaster Pools - stain more easily than vinyl, the layer of plaster is porous to many stains, and rough or etched surfaces accentuate a stain around the edges. Plaster is meant to degrade over time and be acid washed occasionally, to remove a thin layer of plaster, exposing fresh and unstained plaster beneath.
Vinyl Pools - the composition of vinyl makes it less likely to stain in most cases, however organics, metals and minerals can stain and deposit on vinyl liners or the plastic in-pool steps. As a liner ages, it loses more of it's resistance to UV rays and chemical degradation, with fading and staining a more common occurance.
Fiberglass Pools - like vinyl liners, fiberglass pools have a slick and smooth surface that deters some kinds of stains, but they are definitely not stain proof! The gelcoat finish covers the fiberglass shell, and as the pool ages, surface scratches or crazing (small cracks), makes stains more likely to appear. .
Rust stains can develop from construction fasteners that fall into the pool, but more often come from hair pins, aka bobby pins, or pieces of non-pool toys brought into the water. Old water pipes can carry iron into the pool when adding water. A reappearing rust spot on a plaster pool can be a rebar tie wire that is too close to the surface. Chlorine products will cause corrosion of steel when placed near to one another. Vinyl pools with rusty steel coping, or rusty walls have a larger problem to deal with.
Finding the source of the rust, the steel or ferrous material (Iron) is important before removing the stain. First figure out what is causing the rust stain (or any pool stain), to avoid having to retreat the same stain over and over again.
Wearing proper protective gear, fill up an old tube sock with a granular acid like sodium bisulfate (dry acid) - just your regular pH decreaser. You can also use ascorbic acid (vitamin C). Add about a cup of dry acid to the sock, and then tie the sock in a knot to keep the acid contained.
Use your pool pole to position the sock over the stain, holding it in place if the stain is on a vertical wall. Depending on the size and age of the rust stain, the dry acid will eat away at the stain in 5-15 minutes. Brushing with a steel bristled pool brush during and after will help.
If your pool has an overall stain or discoloration, as most do, be aware that beneath the sock, and around the rust stain, you may now have a reverse stain, meaning it may be whiter or brighter than the surrounding areas. Check your sock often to strike a balance between rust stain removal and the area around the stain.
Perhaps the coolest tool ever invented (another one of Andrew Pansini's designs), the Stain Master siphons liquid acid (muriatic acid) from a bottle on the deck, and delivers it to the pool surface, beneath a 3" round cup. It allows you to do underwater acid washing of small areas, and is perfect for removing rust stains in pools.
You can modify a StainMaster with a Frisbee®, to give a larger surface area, for treating larger areas, but for small rust stains, it works great. I've even used a StainMaster on vinyl pools with success, although it's pretty harsh on vinyl to do that (I was desperate at the time!).
As with the sock treatment, keep in mind that the Stain Master can produce a reverse stain, and instead of an ugly rust stain in your pool, you now have an odd looking whiter colored circle, where the StainMaster was placed. This can be reduced by actively and slowly moving the StainMaster in concentrically larger circular patterns around the rust stain.
Vitamin C tablets (with or without Rose Hips, lol), are made of ascorbic acid, which is a mild acid that works well for cleaning. Most rust stains can be removed by rubbing a large Vitamin C tablet over the surface. You can also put 20 or 30 in a heavy ziplock bag and hammer them into a powder, and sprinkle the powder over a rust stain, followed by vigorous brushing.
Stain Free is a pool stain remover that is made from ascorbic acid, already in a granular form, and very useful for treating an entire pool for metal or organic stains. Use 1 lb per 10000 gallons, with low chlorine and high pH (Adding the Ascorbic Acid will lower your pool pH level). Stain Free can also be added to a sock, or poured into a pipe, for concentrated spot treatments.
Place a 1.5 or 2 inch PVC pipe directly over the stain, with the other end of the pipe above water. Now pour in your acid - it can be a dry acid like pH down, No Mor Acid or ascorbic acid, or you can pour in liquid muriatic acid (wear safety goggles and gloves). Hold the pipe over the stain for approximately 30 seconds, and then slowly move it in a circular pattern around the stain, increasing the circle diameter with each revolution.
For rust stains that are on the wall, attach a 90 degree elbow fitting to the pipe, so that you can hold the acid against vertical surfaces.
I mentioned at the start of the article that a persistent, reoccurring rust stain on a plaster pool is likely caused by a rebar tie wire. It could also be caused by the rebar itself. The steel in a pool shell is supposed to be tucked 4-6 inches below the surface, but sometimes bars get bent, or bent improperly, and they end up within 1-2 inches of the surface. After years and years, they could become wet, and begin to bleed rust to the surface.
The only fix for this is to cut out that section of the wall, using a 4" grinder with a diamond blade. Cut right through the rebar and then chisel out to a depth that is beneath the rebar. Coat the rebar ends with rubber sealant, and after it dries, pack the hole full of hydraulic cement. After that dries, top the hole with a plaster mix (guaranteed not to match in color), and then you can refill the pool again.
For plaster or pebble pools that have an isolated rust spot, that is not coming from the inside out, you can use sanding blocks or pneumatic-powered grinder/sanders, to sand down the top plaster coat. Sanding blocks are volcanic blocks, or you can use pumice stone, or our longer-lasting PoolStone Cleaning Blocks and just scrub the surface.
For vinyl and fiberglass pools, stones or grinding wheels are much too rough, but you can use the Stain Eraser for vinyl pools, an abrasive rubber that works like a pencil eraser, to rub off stains from vinyl and fiberglass pools. Easy to use, just attach it to your pool pole and target isolated rust spots or other pool stains.
~ Vinyl Pools need to use a more delicate method, such as the Stain Eraser mentioned above, or Jack's Magic products. Acids can remove rust stains from vinyl, but could damage the vinyl. Jack's Magic The Blue Stuff, the Step Stuff, and the Iron Stuff all work very well for overall rust staining, or for localized rust spots on a vinyl pool.
Thanks for Reading!