4.2. ERMUSR 05-12-2009 - Laserfiche WebLink

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Technical 13~~lletin 1Voo ~~-~~ ;(23 ~aEy 2®~? SelectinS an Appropriate (3vercoat System Adding additional coats of paint to an old paint system will always increase the degree of cohesive stress. The risk factor question is usual{y not if the new system will adhere to the properly prepared surface of the old, it's whether or not the old system has good enough film integrity, adhesion between coats and to the substrate to support an overcoat system without literally falling apart. Before selecting candidate overcoat system(s) one must keep in mind that cohesive stress exerted on the existing system will vary with generic type. Non-flexible conventional 2-component epoxies like Series 27, aliphatic urethanes like Series 1075 and moisture cured urethanes like Series 530 will develop more cohesive stress upon curing than more flexible coatings such as acrylic emulsions (Series 30), medium to long oil alkyds (Series 23) and epoxy mastics (Series 135). One must also keep in mind That wgardless of generic type overcoat system, the higher the film thickness the greater the cohesive stress on the existing system. For example, application of a Series N69/Series 1075 system at i2.0 mils tots! DFT to an old paint system possessing marginal adhesion, would most likely lead to a delamination disaster. Enough cohesive stress will be developed that the old system will be literally pulled apart at its weakest adhesion link (between coats or from the substrate). Geographic location {exposure conditions} also plays a role in the selection of an appropriate overcoat system. Paint systems in the northe--n states undergo more stress because of freeze/thaw cycling. Flexible, low cohesive stress overcoat systems, therefore, become even mere important in those areas of the country that are subject to frequent freeze/thaw cycles during winter months. Delamination Risk Factor: As mentioned earlier, it is important to check adhesion of the existing paint system before deciding whether or not it is a candidate for overcoating. After testing adhesion in accordance with ASTM D 3359 Method A, a delamination risk factor can be assigned on a scale of 1 to 5 with 1 the lowest delamination risk after overcoating and 5 the highest. See Table I, page 5. Worse case scenarios are multi-coat, high film thickness, existing coating systems with poor adhesion between coats or to the substrate in a freeze./thaw locale. A Tnemec overcoat system should not be suggested if a worse case scenario exists. Old paint systems with high delamination risk factors (4.5 to 5) are usually not candidates for overcoating. `vVhen in doubt check with Tnemec's Technical Services Department. Page 3 of 6