An extensive test program has been carried out to assess and compare the pretreatment, E coat and top coat performances of thirteen different coated sheet steels. The products used were all deep drawing, skin panel quality and included various organic coated, electrodeposited and galvanneal steels (with and without flashcoats). The materials consisted of all products currently being used by the Australian automotive industry.
Identical, full size front fenders of each of the 13 steels were processed through the Australian paint shops of Mitsubishi, Toyota, Ford and General Motors Holden's.
Processed fenders of each of the 13 steels, from each of the four car companies, were then tested for phosphate structure, E coat thickness, adhesion and surface quality, and top coat distinctness of image (DOI). Surface waviness testing was also carried out on the unpainted, E coated and top coated surfaces, to establish if any waviness was carried through from the unpainted surface to the top coated surface.
All 13 coated products were cleaned, pretreated and E coated successfully at all four car company E coat plants, despite there being a wide variation in surfaces and phosphate morphologies. Products with organic overlays did not phosphate. No E coat pinhole or cratering defects were obtained on any product. E coat adhesion to the substrate on all products was very high with no failures.
Control of surface prime and top coat paint application variables was difficult when processing non standard panels. Although the difference between best and worst was not great, above average image clarity ratings, as measured by the PGD meter were obtained for most of the top coated galvanneal products (with or without a flashcoat). Conversely, the top coated electrogalvanised products generally produced below average PGD ratings.
Using standard image clarity panels ranging from good to poor, a good linear relationship was found between a visual assessment and the waviness parameters Ra, Rt and RzDIN. Testing of all panels of the 13 products in the unpainted, E coated and top coated condition, using these three parameters produced similar trends to the PGD results:-
- galvanneal coated products in the unpainted, E coated and top coated condition again generally below average waviness.
a wide variation in waviness on the unpainted products was mostly removed after E coating, and further reduced after surface priming and top coating.
The PGD and surface waviness results suggest that the initial unpainted surface waviness has little or no visual influence on the top coated image clarity.