One panel, not the whole door
A sectional garage door is four or five horizontal panels hinged together, and the whole point of that construction is that a damaged panel can be replaced on its own. Reversed into the bottom panel; a cricket ball through a middle one; the bottom panel rusted through on a coastal door while the ones above it are sound — each is a single panel, and replacing the door because one panel is damaged is a waste of a good door. Panel replacement is the repair that is most often sold as a replacement, and it should not be.
The panel is removed by unhinging it from its neighbours and releasing it from the track, the new panel is fitted in its place, the hinges and rollers are transferred or renewed, and the door is balanced and cycled. On a door with a standard profile from a common manufacturer it is a couple of hours. The old panel goes away with the technician.
Matching the panel
The honest complication is the match. Sectional panels come in profiles — the ribbing or raised-panel pattern pressed into the steel — and colours, and a panel from a different manufacturer will not match the ones beside it even if it fits. Where the door is from a manufacturer still making that profile, the match is exact. Where the profile has been discontinued, the options are a close match that is noticeable if you look for it, a panel in the same profile from a different colour that is then sprayed to match, or, on a door where two or three panels are already gone, a conversation about whether the door is at the point where replacement is the better answer.
That conversation is had honestly. A door with one damaged panel gets one panel. A door where three of five are rusted or dented, on a profile nobody makes, is a door that will keep producing panel jobs until it is replaced, and the arithmetic of a new door against three more panel visits is shown to you rather than decided for you.
The coast, and the bottom panel
On the Atlantic Seaboard, the West Coast and False Bay, the bottom panel of a steel sectional door goes first, and it goes from the bottom edge up. Water collects in the bottom rail, salt gets into any scratch or chip in the coating, and the panel rusts through from the inside in a way that shows as bubbling paint long before it shows as a hole. A door in Blouberg or Fish Hoek with a bubbling bottom panel and four sound ones above it is the textbook panel replacement, and it is worth doing before the rust reaches the hinge line, because a hinge pulling out of a rotten panel is how the door ends up crooked.
Where a door is going to face the sea for the next ten years, the replacement panel is fitted with stainless hinges and a coated bottom seal, and the other panels' bottom edges are treated while the technician is there. That is not an upsell; it is the difference between doing this once and doing it every three years.
Bent panels after a cable failure
The other common panel job follows a door that has slewed. When a lifting cable lets go and the door drops on one side, the panels at the bottom twist, and a panel that has been bent even slightly will not sit true in its track — the door binds, the motor strains, and the roller at the bend starts to jump out. So a cable and track repair on a door that slewed hard often includes a panel, and it is assessed and quoted in the same visit so you see both costs separately.
A panel that has been dented by a bumper but not bent out of true — the door still runs straight — is a cosmetic decision. It will not get worse, and it can be left. A panel that has been dented at a hinge or a roller bracket is structural, because the fixing is now in distorted steel, and that one should be replaced before it lets go.
Timber and tip-up doors
A tip-up door is a single panel and does not have panels to replace in the sectional sense, but the same logic applies to its parts: a rotted bottom rail on a timber tip-up can be replaced without replacing the door, as can a cracked panel board, where the frame is sound. Where the rot has reached the pivot points or the frame itself, the door is finished, and that is a new door conversation. The line between the two is whether the frame — the part that carries the springs and the pivot arms — is solid, and it is checked by hand rather than assumed.