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Subject on This Issue:
* Steels & Properties
* Heat Treatment



Comparison Between Water-Based And
Thinner-Based Masking Stop-Off Paints


Masking is an essential part of the carburising process, whereby stop-off paints are used to prevent carbon from diffusing into specific areas of a component. This experiment has been carried out to compare the effectiveness of two types of paints: the conventional thinner-based paint C-90, and a new water-based paint, M-300.

Experiment Design

Four test specimens have been prepared for this experiment. Two of which have been masked with water-based paint (Specimens 1 and 2) whilst the other two have been masked with thinner-based paint (Specimens 3 and 4). Each specimen is then sub-divided into four parts and coated with different layers of the paint; double layers, single layer, dilute layer and no masking, as shown in Fig.1.

Fig.1 Test specimens

Standard dilution factor is used for the single and double layer masking (approx. 10.0% - 12.5% solvent), whereas a dilution factor of approximately 12.5% - 15.0% solvent is used for the diluted layer. The dilution factor for the diluted layer is kept within this range so as to control the consistency of the paint to prevent dripping during drying. These figures apply for both the water-based and thinner-based paints.

After the different layers of the two paints have been applied onto the surface of the specimens, a timer begins recording the average time taken for the paints to dry and harden. Constant intervals of 15 minutes were taken to check on the specimen dryness.

Once the paints have dried, the 4 specimens were subsequently arranged in the cage before loading into the furnace. Specimens 1 and 3 were tied on the top layer of the cage and Specimens 2 and 4 were tied on the bottom layer of the cage.


After the carburising process, test samples were taken from each specimen. These samples were mounted and polished before being tested for surface, case depth and core hardness. The results obtained from the hardness testing are shown in Figs.2-6:

Fig.2 Hardness profile of the portion with no masking

Fig.3 Hardness profile of the portion with diluted layer paint

Fig.4 Hardness profile of the portion with a single layer paint

Fig.5. Hardness profile of the portion with double layer paint

The average recorded drying time for the 2 paints are as follows:
            Water-based paint (M-300)                 -           3 hours
             Thinner-based paint (C-90)                 -           2.25 hours


From the results obtained, we are able to deduce that the conventional thinner-based paint, C-90, has proven to be a more effective stop-off paint for the carburising process. On the other hand, the double layer of thinner-based paint is most effective, followed by the single layer and lastly the diluted layer.

Dilution factor for the water-based paint influences its effectiveness besides preventing dripping. Therefore it is very essential to obtain correct consistency of the paint for optimum results. This might prove to be a very tedious task for the users. Skill and experience is required to achieve the proper consistency, which in turn might be time-consuming. This means an increase in production time, reducing productivity.

The same phenomenon does not apply to the thinner-based paint. The results obtained from the three different layers of C-90 only vary slightly in terms of surface, case depth and core hardness. It is noticeable that the thinner-based paint has very logical hardness profile.. Excellent results can still be obtained from the thinnest diluted layer. This makes it much easier for the users as they may vary the consistency of the paint to their own preference.

There is also quite a major difference in the average time taken for the two different paints to dry. The thinner-based paint takes a shorter time to dry unlike the water-based paint.

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