- 21 December 2016
- Posted by: Stm Coatech
- Category: Educational Articles
Anti corrosive primer applied on ferrous or light alloy surfaces at 4-7 mm dry film thickness. Since first applications were done by washing the object with the primer, they are called “wash primers”. Wash primers are formulated with polyvinylbutyral as well as epoxy and phenolic resins. In the last two decades, wash primers included zinc phosphate pigments instead of zinc chromates due to hazards caused by Cr+6 to the environment and human health. Wash primers are catalyzed with acid-catalysts for a good adhesion on the metal surface. Applying layers with basic resins on the wash primer films with acid molecules left inside may result in intercoat adhesion failure. Therefore, attention should be paid to wash primer application.
Wash primers are typically applied on galvanized, aluminum parts and can be used along with primers that are rich in zinc. Wash primers in general are recommended for a variety of substrates such as:
-Primers rich in zinc
Most wash primers are fast drying and can offer early resistance to rust. They also involve no crucial film thickness and feature low odor and unlimited recoat time. It must be noted that wash primers are not applicable to plastic or wooden surfaces. Wash primer provides excellent adhesion, excellent flexibility, excellent abrasion and impact resistance, very good chemical resistance.
In order to apply wash primers, the following surface preparation methods should be followed:
The surface should be thoroughly degreased and cleaned. The use of pressure washers, steam cleaners and commercial detergents is a must. The surface must be free from detergent residues. It is recommended to use a wiping solvent for the final wipe down.
Objects such as stainless steel and anodized parts should be sanded to loosen the anodized portion and promote adhesion of the wash primer.
For anodized aluminum of architectural grade, the surface should be sanded using 40-80 grit sandpaper until the profile of the surface can be seen. Then, the surface should be degreased and cleaned thoroughly.
In mixing wash primers, it is necessary to mix the base thoroughly until a homogeneous and consistent blend is achieved. Most industries make use of shakers and power mixers, but hand mixing is also acceptable. Once mixed, the converter is added and then mixed again to achieve the right blend and consistency. The most common method to apply a wash primer is through an air atomized spray.
The surface to be coated shall be dry, above 50°F (10°C) and at least 5°F (3°C) above the dew point. All contaminants shall be removed from the steel surface to be coated. Oil and grease should be removed in accordance to SSPC-SP-1. Prepare surfaces by sweep blasting, per SSPC-SP 7, NACE No. 4 or Sa 1. Remove grit prior to coating. To spray Wash Primer, a single-leg, airless spray unit shall be used. The unit shall have a minimum of 30:1 airless pump. A spray tip of 13-17 mils (330-431 microns) shall be used. A wet-on-wet spray technique should be used to achieve a thickness of 5-9 mils (127-229 microns) wet film thickness (0.3-0.5 mils DFT / 7.6-12.7 microns DFT). The coating thickness should be measured using a wet-fi lm thickness gauge. The equipment settings are only guidelines and may vary based on equipment and specifi c application. After 1-2 hours, a topcoat, may be applied.
Allow primer to thoroughly dry before topcoating. All corners and recesses of primed part(s) must be completely dry or poor adhesion of topcoat will result. Use fans on primed parts with detailed geometry to remove solvents and accelerate drying. Depending on conditions, primer may dry in 2-3 hours at 18-24°C (65-75°F). High humidity conditions and low temperatures will slow drying.
- Socomore, Wash Primer, Date of access: 21 December 2016, http://www.socomore.com/telecharger.php?t=11&f=421/pdf//1474987547.pdf&n=c29jb21vcmVfY2hlbWdsYXplLTk5NDRfdXMucGRm
- CorrosionPedia, Wash Primer, Date of access: 21 December 2016, https://www.corrosionpedia.com/definition/1158/wash-primer
- Ecelak Boya ve Kimya Sanayi, 2K Wash Primer Astar, Date of access: 21 December 2016, http://www.ecelak.com/files/ECELAK%202K%20Wash%20Primer%20Astar.pdf