Sink marks are one of the defects that can spoil the finish of a repair in terms of what the customer is able to perceive. That is why the appearance of this defect considerably reduces the quality of the repair carried out and establishes the need to correct the problem.
On the other hand, whenever this type of anomaly is detected, questions are automatically raised such as: why do they appear, or what to do if they do appear?
The answer to these questions requires a detailed analysis of the origin of the problem in order to know how to act and prevent it for future repairs, so this first article analyzes the concept and its causes, leaving the way to avoid and correct the problem for the second post.
Are you ready for this first round? I’m sure you are, so let’s go for it!
What are sink marks?
Sink marks, also known as blemishes, patch marking, or sags, are a finish defect that appear as a mark or imprint on the contour of damage that has been previously repaired, once the various coats of paint that have been applied for this purpose have been fully cured.
The marks formed by this finishing defect manifest themselves to the naked eye in the following ways:
- Generating a striated or curled relief along the entire perimeter contour of the repaired patch. In this case, it is a superficial sink mark.
- Forming a slight step-like inward or outward slope that appears at one or more points of a filled patch. In the most extreme cases, the unevenness covers the entire perimeter edge of the putty patch. In this case, we can define the sink mark as deep.
Why do sink marks appear?
The appearance of sink marks on a repair comes from, generally, the incorrect execution of the priming work carried out prior to the application of finishing paints. In this sense, and following the logical order of work, these are the most common causes of the appearance of sink marks:
Cause 1. Incorrect preparation of the paint edges.
Before filling, it is necessary to open up the paint edges created when removing these coatings with rotary tools. It is a question of cutting out the whole contour of the damage to bevel and attenuate the generated step, so that the putty settles and fills better and, with it, the putty patch integrates perfectly with the rest of the part once it has been sanded.
Therefore, an insufficient degradation of the paint edges hinders a correct filling of the transition zone between them and the metal body. As a result, it is more than likely that small gaps will remain without filler, which over time will cause the filler to give way towards them, causing a slight sagging that will be visible as an outline on the edge of the patch.
Cause 2. Incorrect putty application and sanding.
From the putty application point of view, there are three consequences that can lead to the appearance of sink marks:
- Insufficient hardening of the putty or a poorly homogenized mixture. Either of these two options means that the putty has not hardened sufficiently, so it is exposed to the chemical action of the solvents of the paints that are subsequently applied.
- Excessive application of putty in the perimeter area of the patch. Excessive putty in these areas makes sanding and integration of the patch into the rest of the part difficult.
- Incorrect use of 1k putties. This type of putty is intended to cover small flaws, such as chipped surfaces. Their filling capacity is low, and they tend to shrink a lot, so their application is only recommended in cases where there is no other option due to the state of the repair, that is, just before painting or prior to priming.
On the other hand, improper sanding causes the appearance of sink marks as a consequence of one of these three possibilities:
- Insufficient sanding on the perimeter edge of the patch makes it not integrate well with the rest of the part, causing unevenness that is easily visible after painting. For this reason, it is essential to carry out a precise application of the filler that avoids the accumulation of filler in these areas.
- Excessive sanding. The putty patch must be perfectly level and integrated with the rest of the part, so excessive sanding, especially in parts that have been previously recoated, can cause the edges of the paint to be exposed leading to the appearance of marks in these areas.
- Incorrect sanding of 1k putty. The putty requires perfect sanding, so that, afterwards, it only covers the damage in question for which it has been used. Under no circumstances should this putty go beyond the edges of the damage. Also, it is necessary to make sure that, after sanding, the damage is perfectly filled, and no unevenness or anomaly is noticeable.
Cause 3. Inadequate primer application.
The primer is the undercoat that allows for the sealing and isolation of the putty patch prior to the coating of the finishing paints. One of the primers currently used is of a high thickness, so it helps to cover minor unevenness typical of the repair process once it is sanded, but it is also one that is most prone to sink marks. Therefore, the application of a primer is necessary whenever these cases occur in order to avoid any kind of sink mark:
- When in damages that are not primed, but are sanded, the edges or lower paint layers appear, even when no unevenness is detected in the transition zone.
- When damage has been primed and sanded.
However, sink marks may also occur when the application and drying of the primer is carried out in this way:
- Application of too many coats of primer or in coats that are too thick or too thin. This causes the solvents to take longer to evaporate and may chemically attack the part’s coating.
- The layers of primer are applied in a pyramidal manner i.e. the application starts with one coat covering a larger surface and each coat following inside the previous one, in a pyramid shape.
- The drying time of the primer is not respected, which can cause the solvents in the finishing paints to chemically attack the contour of the primer patch, especially when thicker finishing coats are applied.
Sink marks, the most dreaded defect in primer preparation
The occurrence of sink marks represents a significant loss of quality in the final finish of a repair, in addition to the fact that it can drastically reduce its profitability. For this reason, it is important to know what a sink mark is in order to be able to identify it and find out its causes.
From this point on, the bodyshop professional’s objective is to know what to do in the event of a sink mark in order to effectively correct the problem and prevent it from reoccurring in future repairs, something that will be analyzed in detail in our second post on sink marks, so stay tuned to our social media channels so you don’t miss out on it!
