Source BS 8214:1990 Code of practice for Fire door assemblies with non-metallic leaves
Clause26. Decoration
Unglazed areas of any fire door leaves are generally not required to provide a specific surface spread of flame requirement and may therefore be decorated as desired. There is no evidence to suggest that over painting of heat activated seals has any detrimental effect on the ability of the seals to perform efficiently. There are some benefits in over painting the seals as they are less likely to absorb atmospheric moisture. However, there are limits on how much paint can be applied without there being a risk of the seal being rendered inoperative. It is recommended that over painting be limited to a maximum of five coats of conventional oil bound paint or varnish. When preparing a frame for redecoration, the use of heat or chemical strippers should be avoided if intumescent seals are incorporated. If seals are damaged by either of these processes, they should be replaced in accordance with clause 21. If glazing beads have been painted with intumescent paint, it is essential that they should be repainted with a similar paint.
5 coats of oil paint or varnish times average thickness of a coat of paint 0.5mm with a tolerance of 0.1mm for each coat equals 2mm to 3mm.