The principle of T wash and etch primer is the same however the chemical being used is different.
A-Wash primer is an organic coating that is based on a vinyl butyral resin suspended in isopropyl alcohol. It also contains a corrosion inhibitor, such as zinc chromate
T Wash is a surface pre-treatment method for galvanizing before painting.
T Wash is an etching wash for non-weathered, untreated, galvanised surfaces. It creates a chemical reaction and causes the surface to turn black, which confirms that the surface has been satisfactorily de-greased and is ready for priming.
The purpose of T wash is to remove all traces of contaminants such as oil, grease and dirt. It is not mandatory to use T-wash if the surface is free from contaminants. T-wash will not remove the white salt formation. (White salt formation occurs due to weathering).
Conventional Name of T wash
T wash is being used with the mordant solution and old terminology is being denoted as “Etch primer” or “wash primer” by using PVB (Polyvinyl butyl).
Mordant solution for preparation of Galvanized surface comprise of phosphoric acid and copper salt in a solvent/water solution.
A pre-treatment for galvanised steel before painting. A colour change from grey to black will ensure that the entire surface has been satisfactorily treated.
T-wash is most suitable for application to new galvanizing and should not be used on weathered galvanizing. Hence, it is advisable to minimise the time between pre-treatment and paint application.
Even though this preparation process has been used for some considerable time with varying degrees of success, it is still generally considered to be the best pre-treatment method. The solution is an acidified solution in methylated spirits containing copper carbonate which reacts with the zinc surface.
The degreased surfaces are treated with ‘T-wash’, which is the best brush applied to ensure that the entire surface is coated. The ‘T-wash’ reacts with the galvanizing to form a darkened ‘conditioned’ surface onto which a paint coating can be applied.
Further application(s) can be made to ensure that a total reaction takes place. Care should be taken to avoid ponding or puddling of the ‘T wash’, and any excess should be removed, by washing with water.
Surface Washing After Treatment
Washing down with water brings the need to carry out surface drying before applying the paint coating, whereas not washing down can result in some degree of surface contamination as a residue.
It is generally recommended that water washing and drying should be carried out.
When hot weather conditions are encountered then ‘T-wash’ formulated for tropical environments should be used for the pre-treatment process. High temperatures produce flashing off of the ‘T-wash’ before the reaction process has time to take place, leaving only an apparently reacted surface.
Please note that this is a traditional and conventional pre-treatment method of new galvanized (sheen) before application of duplex coating.
Two-pack etch primers or wash primers are ideal for priming galvanized steel in production line processes where dry film thickness can be strictly controlled and occupational health requirements met. Coatings must be thin, applied by spray and strictly in accordance with the manufacturer’s instructions.
Cold Phosphate Pre-treatment
Cold phosphate pre-treatment is recommended prior to etch priming when maximum adhesion is required. The base vehicle or first pack of a two-pack etch primer contains a polyvinyl butyl resin in alcoholic solvents.
The second pack or activator is an alcoholic solution of phosphoric acid which reacts with the galvanized coating, the pigment and the resin.
When applied, the activated etch primer produces a translucent greenish-yellow film molecularly bonded to the surface of the galvanized coating.
T Wash Primer – Galvanised Metal Primer
T-Wash primer – act as a protective layer (Used as tie-coat over galvanizing) T-wash- pretreatment for galvanised surfaces before painting
The usage of either “T’ Wash or Etch primer is being used as an alternate pre-treatment method by varying chemicals used with appropriate intensity for the purpose of PROFILING to aid the adhesion purpose.
I would conclude in this subject the Wash primer or Etching primer is superior then T – Wash which can guarantee the adhesion to the duplex coating as recorded.
In weathered galvanization removal of white rust is always a big task (high-pressure water wash followed by sweep blasting-chances are high in zinc coating damage).