Painting is the most routine thing done to a door and one of the easiest ways to degrade a fire door. The leaf itself generally tolerates decoration within limits. The parts that must not be painted are the ones that have to move or expand, and they are exactly the parts a decorator working quickly will cover.
What must not be painted
Intumescent seals and smoke seals. Paint on an intumescent strip can restrict its expansion and can bond it into its groove. Brush and fin smoke seals become stiff and stop sealing. Certification labels and plugs on the door edge, which are the only evidence of what the door is, are routinely painted over and then the door becomes unidentifiable. Hinge knuckles and closer mechanisms, which stop working properly.
Coat build on the leaf
Many certifications limit the total paint thickness on a fire door, because a heavy build of certain coatings changes the surface behaviour. In practice this bites in buildings that have been redecorated many times over decades, where the accumulated layers are substantial and the edges of the door have been closed up so far that the gaps are outside tolerance. Stripping back is then part of any remedial work.
Intumescent paint is a different product for a different job
Intumescent coatings are used to upgrade structural steel and, in some systems, timber elements. They are engineered products applied to a specified thickness by a trained applicator with a record of the dry film thickness achieved. Ordinary decorating paint marketed as fire retardant is not the same thing, and neither can be used to turn an ordinary door into a fire door.
Who does the damage and how to stop it
Almost always a decorator with no brief about the doors. The control is to say so in the specification and in the contractor induction: seals masked, labels masked, hinges and closers masked, and a check afterwards. It costs nothing at the time and it is expensive to reverse.
Questions people ask about fire door paint
Can an ordinary door be upgraded with intumescent paint?
Upgrade systems for existing doors exist and they are engineered assemblies with test evidence, not a tin of paint. A general purpose coating does not create a fire door.
How many coats can a fire door take?
The certification states a limit for the doorset. Where a door has been painted many times over decades the accumulated build is a real problem and has to be assessed rather than added to.
What if the seals are already painted?
They are replaced. Cleaning paint out of an intumescent groove is not practical and the seal is a cheap component compared with the door.