Metal Finishing Guide Book


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Page 589 of 903

��� In general, very high solids coatings (near 100% solids content) are not likely to experience solvent entrapment. ��� Coatings that quickly form a surface film, or skin, after being applied are more likely to experience solvent entrapment. ��� Solvent entrapment can also apply to waterborne coatings. ��� Two-component coatings, such as epoxies, polyurethanes must not be applied after they reach their pot life.1 ��� If a two-component coating is close to its pot life and the painter needs to complete the job,2 he must not add thinners to the coating with the expectation that this will extend the pot life. The addition of thinners will lower the viscosity of the coating, but the chemical reactions (cross-linking) that have already taken place cannot be reversed. Therefore, the coating will almost certainly entrap solvents during drying and curing. REFERENCES 1.When the coating approaches its pot life, the viscosity starts to increase rapidly and the painter can no longer achieve an acceptable finish because the paint can no longer be atomized. 2.Many painters are tempted to continue painting even after the coating has reached its pot life. In some cases, it may be possible to mix a small quantity of a new batch and add it to the current batch. This might extend the pot life sufficiently to allow the painter to complete the job. 580

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