Paint fails on vents for one reason. The finish underneath is smooth, and nobody gave the paint anything to hold onto.
The two steps that decide it
Scuff and prime. Everything else in this guide is detail.
A powder-coated surface is a continuous baked skin, closer to a glaze than to a painted wall. It is smooth by design, because smooth is what makes it wipe clean and resist chipping. That same smoothness gives paint nothing to key into.
Scuffing with 220-grit puts microscopic tooth on the surface. A bonding primer then does the chemical half of the job, gripping the slick surface and giving your topcoat something porous to sit on. Do both and the paint stays. Skip either and it will look perfect for a few weeks and then come away in sheets at the first knock.
Keep paint out of the mechanism
The most common avoidable mistake, and it is not about looks.
A floor register has an opposed-blade damper pivoting on a lever. A wall vent has three. An access panel has a cam latch and a concealed hinge. All of those are moving metal with small clearances, and paint is a solid once it dries.
Mask them. A sticking damper is worse than a slightly off-colour vent, because you will be living with it every time you try to balance the room.
Matching the wall
If you are painting a vent or an access panel to disappear, use the actual wall paint, not something close. Sheen matters as much as colour. A matt wall with a satin panel reads as a different colour under side light even when the pigment matches exactly.
When to just buy the right colour
Worth saying, since we sell these. If you are painting a white vent white, you are doing work you could skip. Our wall vents, return grilles and access panels all ship white already.
Painting earns its keep when you want the part to vanish into a colour that is not white, or when you are matching a panel to wallpaper. Floor registers are the other case: ours are brown because brown suits wood and tile, so if you need one in a colour, that is a real reason to paint.
Plastic panels take it too
The plastic access panels are textured instead of smooth, so they actually give primer more to grip than steel does. Same method, same primer, and they will accept wallpaper paste if the wall is papered.
Step by step
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Take it off and take it outside
Painting in place guarantees overspray on the floor or wall and paint in the damper mechanism. Floor registers lift straight out. Wall vents unscrew. Access panel doors lift off the concealed hinge.
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Clean it properly
Degreaser or warm soapy water, then dry it fully. Vents collect a film of household grease and dust that you cannot really see and that paint will not stick to.
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Scuff the whole surface with 220-grit
You are not removing the finish, just taking the shine off so primer has something to key into. The surface should look uniformly dull. Any patch that still shines is a patch where paint will later lift.
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Mask the moving parts
Tape off the damper blades, the pivot and the lever, or the latch and hinge on a panel. Paint in a mechanism is what turns a smooth damper into one that sticks.
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Prime with a bonding primer
This is the step people skip and it is the step that matters. A bonding or adhesion primer is formulated to grip slick surfaces. Ordinary wall primer is not. Thin coat, let it cure fully.
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Two thin colour coats
Thin and twice, not thick and once. Thick coats sag on vertical faces, pool in louver slots, and take much longer to harden.
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Let it harden before refitting
Dry to the touch is not the same as cured. Paint that goes back into a duct too early picks up every piece of grit that blows past, and a floor register that gets stood on too soon will print a shoe.