if if I use enamels as my basecoat, and need to use an enamel as the wash, what type of thinner medium should I use?
Were talking about MM enamels here.

You can use the very same thinner. Just make sure the underlying paint is dry and if you can, apply a gloss finish to "protect" it from the wash. You don't want your wash to ruin your paint right?

Also, make sure you don't cover the model with a wash withthe expectatio nyou'll wipe away the excess. Just dab a bit where you want to emphasize shadow and detail, at the base of raied setails and in panel lines.