A Better Appreciation of Weather

I’m constantly amazed by how little knowledge some people have of the weather, and how it’s likely to change, but I suppose it’s only to be expected if the only time they are outdoors is to walk a few yards between their car and the office. Bikers & farmers seem to have a much better understanding due to being out in it ... and also retired people, presumably due to having more time to look at it.

Despite the wind, rain, etc, spraying outdoors does have some advantages – really – it makes you take account of factors that you just tend to ignore if you’re painting indoors – don’t you assume that the temperature, air quality, and humidity indoors are going to be fine?

Air quality indoors is not always better than out – pollen, insects & the occasional seed are the hazards outdoors; indoors it’s dust (principally skin cells & talc), and human & pet hairs.

Temperature indoors shouldn’t be a problem – but use a thermometer to confirm. Below 10 Celsius is asking for trouble, frankly – paint is likely to bloom/ turn matt below this. I don’t know if there’s really any upper limit for spraying – although temperatures over 30 Celsius (not often a problem in the UK) will mean light coats of paint will dry very quickly, perhaps too quickly when you want to put on a final, heavy coat that will level out & give a properly “wetted” finish...



A hygrometer will measure humidity – and that’s important too, for avoiding bloom/matt finishes. Ideally the humidity should be as low as possible – but I also have to be realistic. I’ll spray primer in anything under 80% humidity, intermediate colour coats in anything under 70%, final colour & especially clear coat preferably at 60% or less. Humidity (also known as dampness) can be an issue indoors, as in the quest to find somewhere out of the way, you could well be looking at cellars & the like ... don’t assume it’s ok.

Air pressure must have a theoretical effect on paint – both the speed it comes out of the can, and the speed it dries, but as at maximum it’s 5 to 8 % either way, I can’t say I’ve noticed. A barometer is still a useful bit of kit though, as it’ll tell you if the weather’s going to change soon, and for the better or for the worse .Falling pressure generally means worse weather (and usually rain) is on the way, rising means better weather.

A 20 GBP “Weather Station” will tell you both temperature & humidity, both on the base unit itself, and on a wireless remote sensor. Basic models often include a barometric sensor to make predictions on weather change, even if they don’t report the air pressure itself.


There’s a lot you can tell about the current weather without any instruments though – if there are puddles on the ground, there’s dew on the grass, soil looking damp, concrete & roads looking wet, etc, then it’s too damp – but if these things are drying out, it means the humidity isn’t that high – and you should be safe to paint once they have.

Concrete that gets wet even though it’s not raining (caused by a rapid increase in air temperature & humidity after a period of cold) means it’s also too humid – and probably will be for several days.

Looking at distant buildings/banks of trees, etc. will tell you something too – if the air is hazy it’s either moisture, or if the sun’s out, it’s probably particulates in the air – both are bad for painting.



Also look at weather forecasts – not the national ones (or even regional ones) on the TV right after the news, but something more specific – in the UK I’d recommend http://www.bbc.co.uk/weather/ as you can put in a specific postcode (usually an area of 15 to 30 premises) & get a heads-up for 5 days ... although thinking you can predict the weather 5 days into the future is just fantasy to me. Up to three days ought to be reasonable accurate, and the next day forecasting certainly should be.

You can also look out of the window on any given day, and if you assume tomorrow will be exactly the same, then you’ll be right 60% of the time – which is about as good a hit rate as professional forecasters, if you ask me.

Written by TB member Jonny Retro

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