A) Sealant applied to a layer of paint

For a good result it is important that:
1 – The paint is cured enough.
2 – The paint has good bonding to the surface.
3 – The painted layer is clean and dry.
4 – The sealant has good bonding to the paint.
5 – The sealant and the layer of paint tolerate each other.

Remarks:
A1 Insufficiently cured paint layers can be weakened by applied sealant (e.g.:. PU sealant).
A2 Bonding of the paint to the surface must be stronger than the “pulling” force of the sealant.
A3 The surface of the paint must be dry. Eventually remove loose dirt. When using degreasers they must not affect the layer of paint. Eventually sanding with fine sanding paper can be executed if this is needed to obtain a clean surface. To achieve a better bonding sanding is not needed, and can even have a negative effect to bonding.
A4 Layer of paint and coatings can be seen as plastics with varying compositions and properties. This can also affect the bonding of sealant to paint. Although the behaviour of sealant on paint concerning bonding is well known, we can only achieve security by doing a compatibility test before application.
A5 Certain sealants and paints are sensitive to weakener migration due to their composition, resulting in a sticky layer between both materials, making good bonding impossible.

With paints this is mainly seen with combinations of acrylate-dispersion paints and Thiokol sealant / polysulfide. These combinations are better not applied.

B) Paint applied to a sealant joint

In this case the elastic sealant joint is painted over with a paint mostly being much less elastic. This can lead to too much pressure on the layer of paint when the joint deforms, creating cracks in the surface. Technically, it is not recommended to paint sealant joints. In the first place this refers to dilatation joints where operation is that big probably each layer of paint will break. Also glazing joints show cracked paint due to this phenomenon.

In building and construction there are two types of joints normally painted due to aesthetics
1 – Connection joints inside
2 – Glazing joints