How to repaint a metal garage door.

A factory garage door finish is baked on, hard and slick. It is designed not to hold anything, which is exactly the problem when you want to put a new colour on it. Roll a coat of house paint straight onto that surface and it will look fine for a summer, then sheet off the first spring.

Calgary adds two accelerants. Road salt — magnesium chloride, active down to about -15°C and harder on steel than rock salt — sits on the bottom panel all winter and undercuts any coating from beneath. And a chinook can move the panel through a 30°C swing in a day, flexing the metal repeatedly while the coating either moves with it or lets go.

This is a short list, and the primer is the only item on it worth being fussy about. A rust-inhibiting direct-to-metal primer is a different product from an all-purpose primer, and substituting one for the other is the difference between a seven-year finish and a one-year one.

Surface rust on the bottom panel is normal on a Calgary door past ten years, and it is a prep problem: sand it back to bright metal, feather the edges, and let the direct-to-metal primer do its job.

Rust you can push a screwdriver through is a different conversation. Once a panel is perforated, coating it just hides the hole for a couple of years. That is the point where the money belongs in a replacement door rather than in paint.

Exterior coatings need roughly 5°C and rising, plus surface temperatures in the same range — a sunny -2°C February afternoon in Calgary does not qualify, no matter how warm the air feels in the chinook. May through September is the honest window.

Watch the sun as much as the thermometer. Spraying or rolling a hot, south-facing steel door in full afternoon sun means the coating starts setting before it levels out, which is how you get lap marks on a surface everyone sees from the street. Work the door in shade, or work it in the morning.

If you have a single door, a Saturday, patience for the sanding, and you are happy with a rolled finish, this is a genuinely doable DIY job. Buy the right primer, do not rush the wash, and it will hold.

The reason people hire it out is the finish, not the difficulty. A garage door is a large flat panel at eye level from the sidewalk, and roller stipple shows on it in a way it never does on a wall. Spraying is what makes a repainted door read as factory rather than refreshed — and spraying well outdoors, with masking that protects the house and the neighbour's car, is where the skill actually sits.