You can absolutely wash your car with bore water and finish spot free, but you need to respect what is in it. Bore water commonly runs 300 to 600 ppm of dissolved solids, two to five times harder than most Australian town supplies, and often carries iron that stains everything it dries on. The fix is the same as for tap water, a deionised final rinse, but the numbers change: the resin works harder, so the tank sizing and the habits matter more. Here is the honest picture for anyone on a bore.
Why bore water is brutal on paint
Bore water is groundwater, and on its way underground it has been dissolving minerals out of rock the whole time. Where a Sydney tap might read 100 to 150 ppm, bores across regional Australia commonly test between 300 and 600 ppm, and some run higher again. Every drop of it that dries on a panel leaves that mineral load behind. On top of the calcium and magnesium, many bores carry dissolved iron, which is why bore-watered fences, walls and cars around the sprinklers wear that orange-brown stain. Iron deposits on paint are ugly, stubborn, and bond harder the longer they bake in the sun.
If you have ever washed the car from the bore tap and watched it dry worse than before you started, that is not your technique. That is the water.
First step: measure it
Do not guess. A TDS meter reading of your bore takes ten seconds and sets every decision that follows: what tank size makes sense, how long a resin fill will last, and whether your bore is in the range where straight DI is the right tool. Every Spot Free Rinse tank ships with a meter, or any cheap handheld TDS meter will give you the number. While you are at it, look at a dried drop on glass. A white crust is hardness, an orange tint means iron is along for the ride.
Can a DI tank handle bore water?
Yes. The purifying resin exchanges dissolved ions regardless of whether they came from a treatment plant or a bore, and that includes dissolved iron. Water that goes in at 400 ppm comes out at 0 ppm and dries to nothing. Three honest caveats:
- Resin life is proportional to what you feed it. Resin that delivers roughly 2,550 to 2,900 litres from a 10L fill at 100 ppm delivers roughly a quarter of that at 400 ppm. The purification works exactly the same, you just consume capacity faster.
- Sediment should not go through the tank. If your bore pulls visible sand or grit, run a basic sediment filter ahead of the tank. Grit does not hurt the chemistry but there is no reason to let it into the vessel.
- Very hard bores plus daily volume is RO territory. If you are well above 400 ppm and washing vehicles every day, an RO membrane feeding a DI polish is the right engineering. Our RO/DI vs DI comparison covers exactly where that line sits. For weekend washing, straight DI stays the simpler tool even on a hard bore.
Sizing a tank for bore water
On a bore, buy the biggest tank you can justify, because capacity is what spaces out your resin changes. Approximate purified output per fill:
| Bore TDS | Medium (10L) | Large (15L) | XL (25L) |
|---|---|---|---|
| 300 ppm | ~850 to 970 litres | ~1,270 to 1,450 litres | ~2,120 to 2,420 litres |
| 400 ppm | ~640 to 725 litres | ~960 to 1,090 litres | ~1,590 to 1,810 litres |
| 500 ppm | ~510 to 580 litres | ~760 to 870 litres | ~1,270 to 1,450 litres |
| 600 ppm | ~425 to 480 litres | ~640 to 725 litres | ~1,060 to 1,210 litres |
A car's final rinse uses roughly 25 to 35 litres, so even at 400 ppm an XL fill covers a season of weekend washes. These figures scale from validated testing at lower TDS, and your bore's exact chemistry will move them, which is one more reason the TDS meter matters.
Habits that stretch resin on a bore
- Purified water is for the final rinse only. Pre-rinse, soap and scrub on straight bore or tap water with the bypass valve open, then switch to the tank for the last pass. This alone multiplies your washes per fill.
- Use rainwater as the feed if you have it. Tank rainwater usually reads under 50 ppm, a fraction of the bore. Feeding the DI tank from the rainwater tank instead of the bore can stretch a resin fill five to ten times further. Best of both: wash on bore water, final rinse with deionised rainwater.
- Rinse efficiently. A steady sheeting pass over each panel does the job. Flooding the car for minutes is resin down the drain.
- Watch the meter, not the calendar. On a bore the readings climb faster, so a five second check before each wash keeps you ahead of it. When it climbs past 10 ppm, the 15 minute resin swap resets you to zero.
Where the rules fit in
Washing on bore water or rainwater also changes what the water restrictions mean for you, since most council rules target town water use. We covered that side in detail in our guide to bore water, rainwater and car washing rules in Australia. This page is the how, that page is the whether.
Frequently asked questions
Can you wash a car with bore water?
Yes, but expect heavy spotting and possible iron staining if it dries on the paint. The reliable fix is a deionised final rinse, which strips the minerals so the car air dries clean.
Will a DI tank remove iron from bore water?
Dissolved iron, yes, the ion exchange resin removes it along with the calcium and magnesium. Particulate rust and sediment should be caught by a simple sediment pre-filter before the tank.
What TDS is bore water in Australia?
Commonly 300 to 600 ppm, though bores vary enormously with local geology. Measure yours with a TDS meter rather than assuming, the reading drives your tank sizing.
How long does DI resin last on bore water?
Proportionally shorter than on town water. At 400 ppm a 10L fill produces roughly 640 to 725 litres of purified water, about 20 car rinses. Bigger tanks and final-rinse-only habits stretch that considerably.
Is rainwater better than bore water for washing?
Much better as a feed. Rainwater typically reads under 50 ppm, so it spots far less on its own and makes DI resin last many times longer when used as the tank's supply.
