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How Hot Is Too Hot To Use Car Touch-Up Paint?

Your car is a temple, ever so beautiful and shiny... Until suddenly, you notice a scratch or a stone chip appear on the paintwork. Alas, that perception is now ruined but is nonetheless easily fixable thanks to the advent of easy to use DIY-style paint touch-up solutions. For just around £30, you can find a plethora of excellent automotive paint touch-up kits online, with everything you'll ever need.

Be it a brush or a bottle, it makes the job of repainting your car far simpler and cheaper than the old-school method of having an entire section be repainted professionally, otherwise costing you hundreds or thousands of pounds in repairs. But there is at least one point of concern that you'll need to worry about as you're pondering over touching up your car's paint - the temperature.

Paints, after all, are very complex chemical compounds that require a specific set of factors for them to be thoroughly applied, in it adhering to your car's paint, drying sufficiently, and being presented nicely. The temperature of the air, surface of your car, and the paint itself play a crucial role, as is the climate the humidity around you. So, just how hot is too hot for touching up your car's paint?

The Goldilocks Zone

But first, let's look into the most ideal temperature you'll need to painting or touching-up your car. The science behind how all these works will look deep into the chemistry of the paint, and its chemical reactions that are necessary for it to cure (or dry), as well as it sticking to the surface of your car's bodywork, and blending in neatly in colour, levelling, and texture to the rest of the untouched paint.

Too hot or cold, and the paint touch-up can interfere with the paint's ability to coalesce and bond to your car. In effect, it can cause the polymer particles to have insufficient energy to move about and bounce around (if the temperature is too cold), and vice versa. Neither is a good circumstance to be painting your car, and we thus recommend that you stick between 6°C to 32°C, just to be safe.

This is a very broad range for ambient temperatures to be applying touch-up paint, and remember that it will vary from one paint to the next. The ideal temperatures to be applying any sort of paint is around 15°C to 25°C, including a relative humidity of the air of less than 85%. In this case, you're best left to do the painting in an insulated, ventilated, well-lit, and humidity-controlled garage.

What Exactly Does Temperature Effect?

Okay, so that's the science out of the way, how does varying temperature affect you while you're in the process of applying touch-up paint? Well, temperature - either too hot, too cold, or just right - will impact the drying times for your paint. More specifically, the drying times for each layer of an alkyd, oil, or water-based paint, which most touch-up paint solutions are made of.

The basecoat, which represents the primary contributor of paint and colour, needs at least two to three or more layers each to get the full effect. This meaning that the paint can sit evenly with the rest of the car - as it's more filled in - and the colour blends in nicely in its hue and texture. Before you can apply a new layer, however, the one before and below it needs to properly dry and cure.

Let's say, for example, that it's too chilly out. This means that the touch-up paint will take more time to dry, so you'll have to wait longer between coats. Instead of around four hours to dry when the temperature is a perfect 20°C, it may take six or more hours if it drops to 6°C or below. It won't just affect your patience, but there will be more than a few imperfections in the paint once it does dry.

Just Make Sure It's Not Too Hot

So far, we can conclude that hotter is better when it comes to painting or touching up your car, as it enables the paint's chemistry to function as intended. However, do remember that painting your car while the temperatures around you are toasty hot is equally a big no-no. In fact, you should avoid painting at all under direct sunlight, or if the ambient temps reach 35°C or higher, and is humid.

40°C or more, and you're really asking for trouble now. With this warmer weather, your touch-up paint will dry too quickly. While you personally will want to move on as quickly as possible to the next coat, the paint needs a bit more time to cure appropriately. In some cases, the drying times for each coat of paint will be cut by half or more, which can be catastrophic to the paint finish.

Here are just some side effects of applying touch-up paint (or old-school repainting) to a car when the climate around you is too sunny and hot...

  • The paint can "flash", meaning that the solvents within the paint can evaporate. When this happens, the succeeding layer of paint will be dissolved by some of the evaporated residue left by the paint below. Consequently, it can form imperfections akin to a splotchy or poorly applied paint job, rather than an even finish all-around.
  • The clearcoat- which goes on top of the basecoat - is usually applied as a thin layer, will dry out even quicker than the basecoat. You'll need to have an adequate mixture and quantity of clearcoat ready on you, as you won't have time to remix it between coats. If you say, run out of clearcoat in the middle of painting a panel, it'll begin to "flash" before you're ready with the next batch.
  • The paint might not stick to the surface of your car. Instead, the paint may peel away and pop off the vehicle after some weeks have passed, as the paint dries out with no actual adhesion to the bodywork or panel below. 

In conclusion, having your car painted or touched up in hot and humid weather isn't just unkind and discomforting to your body (and mind) while you have to work underneath the beaming sun, but it also does terrible things to how the paint will stick, cure, and look. Rather than living with all the defects, do try to stick to that ideal temperature range between 15°C to 25°C for the best results.