I read somewhere that rust (e.g. the bubbly spot on the leading edge of my driver’s door–& along the bottom edge) spreads more when the weather is hot & dry (summer) than during a cold, damp Michigan winter. I’ll hire my collision shop to do the repair so it’s done right; the only question is WHEN–now, or after the winter. Will it make much difference if I wait?
The collision shop could best advise you on whether it’ll matter if you wait.
Actually, rust can form any time the weather is above freezing. The worst conditions are hot, humid, salty air. Hot air can hold a lot more moisture than cold air, and the salt exacerbates the problem. When I was stationed on Guam in the very early '70s very late model cars were already rotting.
I’ll give you a 97% on that answer. 
Rust can form well below freezing (32?). Absolute zero should stop it. It will continue to rust at reasonable rates until the water/Salt mix freezes and then very slowly below that. The fact that it greatly slows down is the reason that there normally will be less rust on a car parked outside in subfreezing weather than in an attached garage where the temperature may be closer to 32?.
What year and model of car? When the paint is blistering, the rust is advanced and the spots you noticed are not the only ones. Save your money for a new car.
In the absence of road salt, the above posts are correct. True, in the tropics, cars rust from the roof down, rather than from the bottom up as they do in the Northern states. The road salt is the villain. Road & Track Magazine a number of years back did a rust survey, and the North East and the area around the great Lakes was 26 times as rust prone as the South West. Cars in Hawaii do not rust as quickly as those in Detroit. Basic chemistry tells us that, all thing equal, rusting increases with temperature. With the addition of sodium chloride and calcium chloride (road salt)the rust races ahead, and rusting will continue well below freezing. In Detroit, the worst place to park your car in the winter is in a heated basement garage. All the accumlated salt will work even faster. The little sea salt that blows on your car in seaside locations is nothing compared to the “bare pavement policy” adopted by many states; they keep adding salt until the pavement is bare. This saves lives, but not cars. So, whatever you read is only true in the absence of road salt. Kurt Ziebart was a mechanic in Detroit where he fixed a lot of rusty cars. He invented the Ziebart System to fight rust, before the manufacturers started doing it.
Press your thumb on the rust and see if it breaks off any of the door panel. If not, you should get it sanded and painted ASAP. Assuming, of course, that the car is worth saving. If it is worth saving and the door has a lot of rust, you can always replace the door. But that’s a bit premature.
There will be far less good metal after Winter is over. If moisture is in there, the freezing could cause more damage.
It’s NOT the COLD DAMP Michigan winters that’s causing the rust…it’s the salt they use on the roads that’s causing the rust. Michigan (especially north of Detroit) is one of the WORSE areas in the country for automobiles. Great for RUST though.
About 20 years the Fisher body commissioned Syracuse University to do a study on why cars rust. They chose SU because the Great Lakes region being one of the snowiest areas in the country was a great place for this study (there actually is a reason it’s called the Rust belt).
Their finding concluded that salt and pooling salty water were the BIGGEST causes of rust. Their recommendation was to have as few places that water can pool as possible. Protect all exposed metal. Since then almost every car manufacturer has been following these recommendations and the number of cars that rust out is a factor of 100 times less then it was 20 years ago.
To answer your question of WHEN, the answer is NOW. It does not matter whether it is winter or summer. Time is rusts greatest ally. The longer you wait, the worse it will get and the more expensive it will be to repair.
Still wondering then… I live in NY. I’m buying a new Prius next week. Should I keep it in the attached garage or park it outside. [I have kept my 1990 Camry in the garage for 17 years and there is a fair amount of rust in the fenders behind the front wheels and around the edge of the rear wheelwells.]
I would probably keep it in the garage, provided the garage is not heated. I keep a thermometer in the garage and in the winter, the temp. is just below freezing. An ideal situation. If you keep the car outside, you will need a $3000 paint job much earlier, and all the mechanical components will wear out quicker.
So, your question is very valid; and if you wash the Prius regularly and rinse the wheel wells, you will likely get a very long life out of your Prius body. Also, Toyota rust protection has improved greatly since you 1990 Camry was built!!
Docnick – Thanks for your advice/help. I’ll keep it in the garage. I usually wash a new car frequently when I first get it. Then gradually taper off over the next year or so. I’ll try to at least wash off the road salt.
All Camrys of that vintage rusted there…including my 1991!
Use the garage without worry. Rust prevention is designed into cars today rather than added on afterwards. Body shells are designed to vent, to drain, and to not contain cavities accessible to water. Alloys and polymers that do not promote rust are used, and metals are electrolytically and conformally coated to prevent exposure. Seams are bonded using bonding agents and techniques such as ultrasonics that do not leave “heat affected zones” to the extend that old seam welding did. Cars are simply designed now to prevent rusting. It simply isn’t the issue it once was.