A wash primer (or shop primer) is a pre-treatment product used as first coat for metal surfaces. It is specially designed to offer support, consistency and adhesion to all types of paint. Often the wash primer is followed by a priming base coat to further increase the performance of the final paint coating.
The wash primer is applied by spray on well cleaned and degreased surfaces, sometimes even sanded (steel and anodized aluminum) to make them more reactive and provide greater binding to the coating. The use of wash primer is suitable for all those sectors in which enormous resistance to corrosion, time and wear are required, such as automotive, aeronautics and naval.
These products contain a high percentage of resins in alcoholic solution, capable of drying and reticulating quickly after application. The spray by atomizer allows a correct deposition of the wash primer and a reactivity combined with the acid contained in the product and the oxygen in the air. In addition to resins, there are fillers, rheological additives, minerals, dyes. The acid (phosphoric) is usually added separately just before application and is responsible for activating the resins and improving adhesion to the metal.
After 15-45 minutes
from application, the applied layer of wash primer is completely dried and can
be safely painted or lightly sanded to further promote the coating adhesion.
The resin most commonly used is polyvinyl butyral (PVB), which has elastic and hydrophobic characteristics, with a rapid and tenacious degree of cross-linking. This resin can be used alone or in synergy with phenolic or epoxy resins, or it can be completely replaced by the latter, depending on the application area. The solvents used in the formula must be suitable for dissolving the resins (especially the PVB), for maintaining a correct viscosity and optimal evaporation after application; alcohols are often used such as: methoxypropanol, iso-butanol, xylene (pay attention during application). The total percentage of resins can exceed 25%. This quantity is useful, at the same time, to keep the mineral component and the various additives and pigments in suspension. Minerals are used as fillers, anti-sticking agents, protective against wear and scratches, opacifiers or whiteners (such as talc) while as additives products based on zinc phosphate are particularly used (sometimes in synergy with barium borate), with the function of corrosion inhibitors (replacing the old toxic chromates). Other corrosion inhibitors, completely non-toxic, are now widespread, such as, for example, magnesium oxy-amino phosphate. Typical quantities of mineral fillers in the formula vary between 5 and 12%, while for inhibitors between 2 and 5%. The formulation is completed by: colored pigments (if required), rheology additives, surface modifiers and suspending agents (silica and similar) in small quantities (0.5 - 1.5%).
primers are two-component (or 2K) products, as the acid catalyst (mostly
phosphoric acid) is added and mixed well before application (in the ratios
described by the supplier). Once added, the product has a duration of a few
hours, necessary for use. There are also one-component wash primers
(1K), containing resins with different chemistry (such as modified
self-crosslinking epoxies) or balanced ratios between resins and activator so
as not to react until use.
1K transparent wash primer example
- PVB (15% in alcohol solution): 65-70%
- Epoxy: 2-3%
- Phenolic: 4-5%
- Anticorrosive additive: 2-3%
- Talc: 4-5%
- Rheological silicas: 1-2%
- Phosphoric acid 85%: 0.5-1%
- Solvents: 10-15%
2K wash primer example
- Phenolic: 10-12%
- PVB: 8-10%
- Coloring pigments: 3-4%
- Anticorrosion additive: 4-5%
- Talc: 4-5%
- Rheological Bentonites: 1-2%
- Solvents: 60-65%
Additive: 7-8% phosphoric
acid (to combine 4: 1)