What form is developer NOT commonly available in for penetrant testing?

In penetrant testing, developers play a crucial role in highlighting surface defects by helping the penetrant to bleed out from any discontinuities and become visible. While developers are available in various forms to suit different applications, high viscosity developers are not commonly used.

Developers are primarily utilized in aerosol spray, powder, or liquid form. Aerosol sprays facilitate easy application and uniform coverage over the surface being tested. Powder developers are likewise common, as they can be dusted onto the surface to absorb any excess penetrant. Liquid developers are also widely used; they can penetrate the surface and provide a clearer indication of defects.

High viscosity developers would be challenging to work with since their thickness could hinder proper penetration into small defects. This would limit their effectiveness in fluorescent or color-contrast penetrant testing, making them an uncommon choice in standard practices. Hence, out of the options listed, high viscosity developers are not typically available for penetrant testing.