Touch-up paint can, in a way, be considered a miracle of modern engineering and chemistry. In the old days, you'll have to get your entire car - or a whole panel - repainted professionally just for a tiny crack or chip. Now, there are dedicated repair kits that you can easily use at home. Yet, there are a few limitations to touch-ups, and one of them is something that we don't consider often - weather.
The chemicals inside touch-up paint are just as complex as paints typically are. Thus, they need a set of unique circumstances to be properly applied, and the climate that it's exposed to while the touch-up is happening is one of those factors. The humidity and temperature are the most important ones to think about anytime you're painting your car, be it a small bottle and brush, or with a spray can.
But what if you have no choice? What if it's rapidly approaching winter soon - while the air around you is slowly dropping into crystalline coldness - and you need your car's scratches and chips covered up to prevent damage, or even worse, rust? Rust, of course, can easily eat into the now-exposed and unpainted layer to corrode the bodywork of your car. How cold then, is too cold for paint touch-ups?
For Paint - Hot > Cold
Firstly, what's the science between the effects of temperature on how a paint properly cures - that is to say, sufficiently dried and "painted" onto the bodywork or panel? In layman's terms, this involves knowing how the compounds in the paint are able to dry themselves, and ensuring that the necessary chemical reactions occur smoothly so that the newly-applied touch-up can adhere to the car's bodywork.
More importantly, it needs to "blend" in with the rest of the original, untouched paint. Generally, we can say that hotter temperatures are far better than colder weather when it comes to using touch-up paint. Depending on the touch-up solution that you're using, the recommended temperature to let it cure will vary. But on average, an ambient temperature between 6°C to 35°C is okay enough.
Ideally, try to stick to around 15°C to 25°C with most touch-up paints, which is the best for painting or touching up your car. That, combined with a humidity of less than 85% is a healthy balance between letting it cure sufficiently, but neither too quickly nor slowly. Thus, you're best left to touch up your car's paint in a well-lit, insulated, heated, humidity-controlled, and adequately ventilated garage.
What Happens If You Paint Your Car While It's Too Cold Outside?
But what if you don't have a garage, and it's getting too cold outside? Could you still make do with a touch-up in a pinch anyway? Well, it depends. You see, colder temperatures won't enable the paint to dry quickly enough, and slows down the overall drying time for each layer or coating of alkyd, oil, or water-based paints (which most touch-up solutions are made of). Remember, you need several coats.
Cold conditions can interfere with the paint's ability to coalesce and bond with your car. If it's too chilly out when you're applying the paint - especially so if the surface that you're painting is rock solid - the polymer particles in the paint don't have sufficient energy to move around and bounce together. Thus, the temperature of the surface, paint, and air need to be ideal for it to "stick".
For the basecoat - the primary base colour for the paint - you may need two to three (or more) coats on top of one another to get the full effect, and thus levelling out your paint to match the rest of the car, while also making sure the colour matches and blends nicely. And before you can add the new layer, you also have to make sure that the layer of touch-up paint below that one dries thoroughly.
When it's cold out, you'll need to wait longer before applying another layer. For example, if there's a more ideal ambient temperature of around 20°C, you may have to wait around four hours for each layer to dry - though this will vary depending on what paint touch-up you're using. But if the temperature drops to say, 6°C or below, it might take six or more hours for that layer to dry.
This effect is worsened if you use oil-based paints, which are then applied with a spray can or gun. This way, the layer of paint won't be able to dry (or cure) properly, and there will be one or several of these side effects as a consequence:
- Poor colour uniformity between the touched-up area and the rest of your car's paint.
- Water spotting might appear, which are whitish spots or blemishes on the surface of the paint caused by exposure to moisture.
- Bubbles or bumps might be created on the paint surface once applied.
- Paint may, rather than dry, runs and drips off the body surface.
- The spray gun might also fail to function in dispensing the paint and may get clogged in colder temperatures.
- Blisters can form, and the paint may not adhere properly, resulting in you needing to repaint or retouch it up again.
Some Dry Faster Than Others
So far then, we know now that temperature is crucial when it comes to deciding as to the when and where of getting your car's paint touched up. Overall, let's try to avoid painting your car at all when the temperature drops to 6°C or less around you - although some paints are more sensitive, and need at least 15°C to dry properly. It's worth noting as well, that paint pens and brushes dry quicker.
Spray cans or guns may make the painting job easier over a larger surface, but its wider application does come at the cost of drying times. Therefore, you can more comfortably use paint brushes or pens - which most touch-up kits come with, anyway - in colder climates compared to a spray gun. Another important thing to look at is the clearcoat, which goes on top of the basecoat.
This thin and transparent layer of paint aids in protecting the colourful basecoat below it from any debris or damage. Clearcoat also needs to cure adequately, and cold temperatures can have a huge - arguably bigger - impact on the drying of the mostly urethane-based paint. The entire curing process can slow down, or stop entirely if the temperatures around you drop to below around 15°C.
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