| | | quote | | Originally posted by rogergarrison: ... I shoot some white over primer when painting a yellow car. I find that for some reason none of the paint suppliers can explain, that paint will peel from gray primer over time but will not peel off of red oxide or black primer. Ive seen this as a problem for 40 years. You will usually find that plastic parts have a water based primer that is black and only needs scuffed up to paint over, and no other primer is recommended. |
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Some engineering comments:
Both GM and Ford have had well-known and expensive problems over the past decade with paint peeling due to failure of the primer caused by UV (ultraviolet) light exposure. Sunlight is by far the major environmental source of UV light.
In general, primer depends upon the color coats for protection from UV light, which otherwise may cause primer to slowly decompose over time. Some primers are more UV-sensitive than others. Conversely, and highly simplified, color paint is a mixture of ultraviolet-resistant resins and powdered pigments. The resins themselves are clear (or nearly clear), so light will easily pass through them. On the other hand, the primary job of the pigment is to efficiently reflect the desired color of light, and a secondary job is to absorb all other colors of light and thus prevent it from reaching the primer coat. Some paint colors (i.e. some pigments) are better than others at blocking light from reaching the primer; black is by far the best absorber of all colors of light, including UV, and most whites containing titanium dioxide are pretty opaque as well. But when shooting a color like yellow, which as Roger indicated is generally one of the worst in this respect, you may need to 1) use a UV-resistant primer or 2) apply more color coats than usual ... or both.
Many plastics, including fiberglass body panels and the structural plastics used in Fiero body panels, will also decompose upon long-term exposure to UV light. Probably the main reason that GM used black primer on our Fieros was to provide a final UV barrier to protect the plastic body panels from any UV light that makes it through the color coats. Use of black primer is gospel among the builders of composite (e.g. fiberglass/plastic) aircraft, where deterioration of painted structural components can have serious consequences.
For what it's worth, different plastics have vastly different tolerance of UV exposure. Expoxies are generally among the worst (and almost always have to be painted), while two-part urethanes are often among the most UV-resistant.
For Fiero (or Corvette) body panels, probably the best plan for paint longevity is to use black primer, optionally followed by a thin white color coat to brighten the final color, followed by the desired color coats and optional clear coat(s).
[This message has been edited by Marvin McInnis (edited 06-23-2005).]
