פירוש על עבודה זרה 148:3

Others say, Raba says only when tarred: [if the vat is tarred, but] he did not trample [grapes] in it, this is the case in which scouring is sufficient.

Cleansing it is not sufficient - rather, peeling [off the pitch entirely is necessary] because wine may enter the cracks in the pitch.

According to this version of Rava’s statement, Rava is teaching that scouring is sufficient only if the non-Jew did not also tread his grapes in the pitch-lined winepress. If he also tread grapes in it, the wine is more absorbed and scouring is not sufficient. This is a more stringent version of the teaching.