How to wash a rash guard

A rash guard is not made of one thing, and it does not fail as one thing. The bulk of the yarn is a synthetic filament, usually polyester, doing the job of holding a shape and taking abrasion. Threaded through it is a much smaller percentage of elastane, the fibre also sold as spandex and under the Lycra trademark, doing the job of returning the fabric to its original dimensions after it has been stretched.

Those two components fail in different ways and on different timescales. The polyester loses surface, pills and eventually thins where it is abraded. The elastane loses return. When people say a top has gone baggy at the elbows, or that it no longer sits flat across the shoulders, they are describing a loss of return, which depends mostly on the small fibre and on how the fabric is built around it.

That distinction decides what is worth caring about in a wash cycle. It also explains why so much laundry advice is unfalsifiable: it treats the garment as one material and then attributes any change in it to whichever variable the writer wanted to talk about.

Two pieces of published work do most of the useful work here, and neither of them tested a rash guard.

Jovanovic and colleagues, writing in Materials in 2022, knitted plated weft fabrics from polyamide with elastane percentages ranging from zero to forty-three per cent, then cyclically loaded them and calculated unrecovered elongation, elastic elongation and a hysteresis index. Their result was that the elastane percentage significantly affects the size of the elastic region of the fabric, the range within which unrecovered deformation completely disappears, and has no effect on the hysteresis index. Their conclusion was that the elastane percentage has to be optimised for each fabric design rather than simply increased.

Sular and Oner, in Fibres and Textiles in Eastern Europe in 2019, took the question directly to the washing machine. They put twelve elastane knitted fabrics through repeated home laundering and measured cyclic deformation after zero, five, fifteen and twenty-five wash cycles at four different recovery times. They report that five and fifteen cycles were the significant points across their fabrics, and that fabric construction, specifically the viscose ground yarn and the polyamide and elastane gimped yarn, changed how much residual extension a fabric was left with.

Read those two together and the finding that matters is not about temperature or detergent at all. It is that ordinary home laundering changes a stretch knit's recovery, that the change builds over repeated cycles, and that the fabric's own construction determines how much.

One honest limitation. The Jovanovic fabrics were polyamide and elastane; the Sular and Oner fabrics were viscose with polyamide and elastane gimped yarns. Neither is the polyester-dominant knit that most rash guards are made from, and neither study was designed to describe one. They tell you which property to watch. They do not tell you what your top does.

The usual instruction is to skip the tumble dryer and hang the garment up. We could not find a study that measured tumble drying against line drying for this fabric class, so that instruction is not something this article can support with evidence.

What has been measured is the other end of the same choice. A 2024 study in Fibers and Polymers subjected warp-knitted swimwear fabric to artificial weathering for 120 and 240 hours and measured breaking force, elongation at break, air permeability, fabric elasticity and stiffness, and dimensional characteristics, alongside electron microscopy and infrared spectroscopy. Ultraviolet exposure was the dominant factor: breaking force, elongation at break, elasticity, stiffness and air permeability were all strongly affected, micro-damage was visible under the microscope, and no chemical destruction showed in the infrared spectra. The authors also tested seawater treatment and found its effect usually minimal or not distinctive.

That is a study of sun exposure over hundreds of hours on swimwear, not of an afternoon on a balcony. It does not license a rule. What it does establish is that ultraviolet light is a real and measured degradation pathway for this family of fabrics, which puts a question mark over the most common piece of advice in the category, and means that hanging a stretch garment in direct sun to dry it is not obviously the conservative option it is usually presented as.

The complaint that survives the wash is usually smell, and the research on it points at the fibre rather than the cycle.

Callewaert and colleagues, in Applied and Environmental Microbiology in 2014, collected T-shirts from twenty-six people after an intensive indoor cycling session, incubated them for twenty-eight hours, and had a trained odour panel assess them. The panel found significant differences between polyester and cotton on the hedonic value, the intensity and five qualitative odour characteristics: the polyester shirts smelled significantly less pleasant and more intense than the cotton ones. The paper attributes the difference to differential microbial growth on the two fibre types, reporting micrococci in almost all the synthetic shirts and on almost none of the cotton ones, with staphylococci abundant on both.

This article makes no hygiene, health or infection claim on the back of that, and the study makes none either; it is a study of odour development and of which organisms grow where. The practical reading is narrow and worth having anyway. If a synthetic top smells after training and a cotton one did not, that is a documented property of the material rather than evidence you washed it wrong, and no washing routine changes what the fabric is.

Four claims that appear in nearly every published guide to washing stretch sportswear were cut from this article, and it is worth naming them rather than quietly leaving them out.

A specific maximum wash temperature. There is a great deal of confident writing about thirty degrees against forty, and we could not find a study that tested wash temperature against elastic recovery in this fabric class. The one laundering study above did not compare temperatures.

Detergent chemistry. The claim that alkaline or enzymatic detergents attack elastane specifically is repeated everywhere. The laundering study did not compare formulations, so we have nothing to report.

Chlorine. Chlorinated water is frequently blamed for elastane failure. One 2024 study, by Potočić Matković and colleagues in Polymers, left nine polyamide- and polyester-elastane swimwear knits in a chlorinated outdoor pool for 200 and 300 hours. Breaking force fell by 12.4 per cent after 200 hours in chlorine and by 65.7 per cent after 300, while breaking elongation barely moved. That is a measured strength loss from long immersion. It is not a measure of recovery, and hundreds of hours in a pool is not a training top's exposure, so it does not support a rule about washing.

Fabric softener. We found finishing studies on softeners and moisture management in knits, but none on household softener and elastane recovery.

None of those four is a laundry rule we can give you, and none of them is refuted by the absence of evidence either. The point of listing them is that the writing you will find elsewhere states all four as fact, and that should tell you something about how it was produced.

The most useful document you already own is the label, provided you read it as what it is rather than as advice.

The care symbol system is administered by GINETEX, whose five symbols are protected trademarks in most countries and which describes itself as the origin of the ISO 3758 international standard on care labelling using symbols. The fourth edition of that standard, ISO 3758:2023, was published in December 2023 and replaced the 2012 edition.

Two things GINETEX says about the symbols change how you should read them. The symbols indicate the maximum permitted treatment, not the recommended one, which means gentler handling than the label shows is always allowed and the number in the wash tub is a ceiling rather than a target. And care labelling is not a guarantee of quality: the label tells you what the manufacturer says the garment will tolerate, and the standard governs how that is expressed, not whether it is true.

There are also published test methods behind the properties a manufacturer would need in order to know. ASTM D2594 covers stretch properties of knitted fabrics having low power and measures both fabric stretch and fabric growth, the residual extension left after the load comes off. AATCC publishes colourfastness test methods, including one for chlorinated pool water, TM162. Almost nobody publishes results from any of them, and that absence is why the checks you can run yourself carry more weight than they should have to.

The measured variable is the number of wash cycles, not the ones people argue about. Recovery declines with repeated home laundering in the one study that tested it, and construction changes how much. Ultraviolet exposure is a documented degradation pathway for related fabrics. Odour is substantially a property of the fibre. Everything else in this article is a gap in the record rather than a rule.

We are not going to tell you that any washing routine sanitises anything, prevents anything, or extends the life of a garment by a number of months, because we have no evidence for any of those sentences and neither does anybody selling you a detergent.

If you want one thing to take from this, take the reframing rather than a rule. Each wash spends a little of a finite property, and the property being spent is return rather than strength. That is worth knowing when you decide how many training tops you want in rotation, and it is the only part of this that the evidence actually supports.

Correction, 29 September 2026: an earlier version of this article said the laundering study washed at forty degrees with a mild detergent, and that we had found no study of chlorine. The publisher's copy of the laundering paper has gone, and the method details could not be checked against what remains, so they have been removed. A 2024 chlorine study has since been found and is described above.