Telegraphing is a defect which occurs as the reflection of the coated surface texture on the upper surface of the paint after curing / drying.
Reflection of fingerprints, water stains or sanding marks on the painted surface is also called telegraph defect, but these defects are usually visuals caused by the dewetting of the surface. Normally, telegraphing is caused by surface tension differences stem from differences in temperature or concentration on the surface or part of the surface. For example, during the baking of metal surfaces in the form of sheets, reinforcement elements, if any, may cause temperature differences because they act like thermal pits. In this case, the temperature of the area above the reinforcement elements is lower than the sections in the form of thin sheets, and consequently the surface tension of the paint in this part is higher.
As a result, a surface tension change occurs in the paint and therefore the paint stacking towards the part where the reinforcement element is located. This causes the shape of the reinforcement element at the bottom to appear on the surface, the presence of the reinforcement element is notified by “telegraphing” above. A similar result can occur when the panel is baked not by hanging, but by placing it on an perforated oven tray. In this case, the shape of the tray holes can form on the surface of the cured film. A telegraph image of temperature differences can be seen in Figure 1.
(Fig. 1. Basecoat aluminum show different orientations due to the different heating/cooling properties of the sealants applied to the door support profiles compared to the steel, the color change in these regions.)
The paint applied to the surface after sanding process completely fills the indentations / protrusions formed on the surface, change the topography of the surface and affect the appearance according to the non-sanding area. At these points, gloss may be seen lower, and orientation differences can be observed in metallic paints.
(Fig. 2: The image of the lower layer moved to the upper layer due to the surface tension differences that occurred after sanding the lower layer)
Fig. 3. — Reflection of a fingerprint formed on the lower layer to the upper layer
Silicones are effective in preventing telegraph defect as they reduce surface tension by reducing surface tension differences. However, it should be kept in mind that in the case of using silicones that are very effective in reducing the surface tension, the tolerance to large surfaces will increase and accordingly, the roughness on the application surface (e.g. sanding marks) will reflect more on the paint surface. In cases where this reflection disturbs the visual expectations, the use of acrylic surface agents can also give positive results. Increasing the coating viscosity is also very effective in reducing telegraph defect. In many ways, defects in the form of paint accumulation on the edges and the appearance of a frame are similar to the telegraph defects. Therefore, most of the actions to correct the defect of frame appearance are also effective against the telegram defect.