MIG with shielding gas is probably one of the most finicky in regards to cleanliness.
We used to weld through the varnish/paint that comes on pipe from the mill. When we used Stick and FCAW the welds worked fine, but MIG would look like bird crap.
I'm not condoning doing this, the best practice is good clean base metals. I am just saying what I have personally experienced.
I'd suggest taking a torch and pre-heating the area you are going to weld to 400 F. That should help drive off moisture and contaminates.
