An overall wash is done on a flat surface-it imparts a grimy or dusty look.
A pinwash is done on a gloss finish-the gloss enables the mix to flow into psanel lines and around detail.
You can do one or both if you like.
Enamel or oils make good washes,They should however be applied over a thoroughly cured acrylic clear coat.
These are just some fast answers,there a dozens of diffrent techniques.Look for books or magazines giving a more comprehensive overview of diffrent styles.Or Google "weathering technique" articles for more info. Mig Products,AK,Vallejo all detail diffrent styles.