en.mi piel/en la piel ... wouldn't the definite article be more appropriate or is piel an exception to the rule? thank you

Hola Emanuel,

both are fine, you can use la or mi. Using "la" would be applying the same rule as for "Me lavo los dientes", no need to use the possessive as it is obviously your own teeth. In this context (routine/hygiene) it would sound odd to use the possessive. 

Here we are also talking about feeling the breeze on you own skin. With la/mi piel, both can be used though. Using the possessive may give it a bit more emphasis. 

Saludos