Repainting a wooden garage door in Calgary.

A steel door is dimensionally stable; it expands and contracts, but it does not absorb anything. A wood door takes on and gives up moisture with every weather change, swelling and shrinking as it goes. In Calgary, where chinooks push the temperature 30°C or more inside a day and the humidity moves with it, that cycle runs constantly.

Everything about the coating choice follows from that. The primer and the finish both have to stay flexible, because a brittle film on a moving substrate cracks — and every crack is a route for water into the wood underneath. It is also why a wood door here realistically gives five to seven years between repaints, where a properly primed steel door gives seven to ten.

The single biggest difference between a wood door repaint that lasts and one that fails is whether the edges were sealed. The face you see is easy. The top edge, the bottom edge and the sides of each panel are the ones that get skipped, and they are exactly where end grain is exposed.

End grain behaves like a bundle of straws. Left unsealed, the bottom edge wicks up meltwater and salt slush all winter, the wood swells, and the coating on the face lifts from below. If you only take one thing from this page, seal the bottom edge.

Most Calgary wood doors we look at have some combination of the same three things: a grey, dry patch on the sun-facing lower panels, a split or two where a panel has moved against its frame, and soft wood along the bottom rail.

Grey and dry is fine — sand it back to clean wood and prime it. Splits get filled and sealed. Soft wood is the one to look at honestly: if you can press a screwdriver into the bottom rail, that section needs a wood repair or replacement before it is worth painting, because paint over rot just makes rot you cannot see.

A stained wood door on a Calgary elevation is beautiful and needs more attention. Semi-transparent stains give up their UV protection faster at 1,100 m with 333 sunny days a year, so a stained south-facing door often wants a maintenance coat every three to four years.

Paint hides the grain but holds much better. If the door is already painted, going back to stain means stripping it, which is rarely worth the labour. If it has never been coated and you want the wood to show, plan on the shorter maintenance cycle before you commit.

May through September, above 5°C, with the wood dry going in. Wood is less forgiving than steel about damp: a door washed in the evening and coated the next morning may still be holding moisture in the end grain, and that moisture has nowhere to go once the film closes over it.

Give yourself two dry days if you can — one to wash and prep, one to prime and coat. Then keep the door closed four to six hours after the final coat so the fresh edges do not stick to the weatherstripping.