Interior aftercare: sun, heat and leather

How hot did parked cars get in an Arizona study?

In a 2018 study by Arizona State University and the UC San Diego School of Medicine, run over three hot summer days with temperatures in the 100s in Tempe, Arizona, cars parked in the sun averaged 116 degrees inside after one hour. Dashboards averaged 157 degrees, steering wheels 127 and seats 123. For cars parked in the shade, interior temperatures were closer to 100 degrees after one hour, with dashboards averaging 118, steering wheels 107 and seats 105.

What does sun do to a dashboard, vinyl, plastic and cloth?

Gold Eagle says that on dashboards, UV exposure weakens the polymers that keep plastics flexible, which can leave them chalky, brittle, warped or cracked over time. It also says repeated heating and cooling cycles stress adhesives, coatings and layered materials, and that dust and grime can trap heat and make surfaces degrade faster.

Fibrenew says UV rays can cause vinyl to become discolored, brittle and prone to cracking, and that because vinyl has no natural oils it can deteriorate more quickly without proper care. Fibrenew adds that fabric seats, while generally more resistant to temperature fluctuations, can still suffer from sun damage: prolonged sun can make fabric fade, weaken and become threadbare.

What does conditioning do, and what wears leather down?

Leather is a natural material. Dr. Beasley's compares conditioning leather to using lotion on dry skin and says it keeps leather hydrated and flexible, adding that many leather conditioners include UV protection, which helps prevent fading from sunlight.

Auto Detail Doctor, a detailing business whose article promotes Leatherique products, names the two biggest causes of leather damage as dirt and debris, which act like sandpaper, and oils from skin, which break down the surface coating. The same article lists sweat, lotions and sunscreen among the contaminants that break down the protective coating.

How often is leather conditioning suggested?

The suggested frequency depends on who you ask. Dr. Beasley's suggests every three to six months, and breaks it down as once a year for low-traffic leather, every six months for medium traffic and every three months for high traffic. Auto Detail Doctor gives a rule of thumb of every 45 to 60 days for most vehicles, more often in dry or extreme climates. Fibrenew says to clean and protect your seats 6 to 8 times a year.

Dr. Beasley's says the right interval varies with how often you drive the car and what sort of traffic the leather sees.

What do window glass and a sunshade do about UV?

The Skin Cancer Foundation, writing about skin exposure, says window glass does a good job of filtering most UVB rays, but UVA rays pass through windows and clouds year-round. Three Rivers Auto Glass, an auto glass company, says modern windshields have a laminated interlayer, often made of polyvinyl butyral (PVB), that it describes as giving up to 99 percent protection against UV rays.

Gold Eagle says a windshield sunshade reduces direct exposure on the dashboard, steering wheel and upper seat surfaces, and Dr. Beasley's also suggests using a sunshade to protect car seats from direct sunlight.

What signs of wear do the sources list?

Dr. Beasley's lists signs that leather needs conditioning: it feels dry or stiff, small cracks or creases form, or it looks dull or faded. Gold Eagle lists signs of UV wear: a dash that looks faded, gray or powdery, leather that feels stiff or loses its natural color, and plastics that feel dry, brittle or overly glossy.

Gold Eagle also lists small cracks, peeling, curling edges and warped trim among the signs, and says restoration becomes much harder once they appear.