opposite of hobnail?

There is no opposite for hobnail.

http://dictionary.reference.com/browse/hobnail

I doubt there's an opposite (antonym) for this particular word -- unless there are other ways of attaching certain types of materials to others.

The opposite of hobnail is a smooth or plain surface. To find the opposite of a word, you can use a thesaurus, which is a reference tool that provides synonyms and antonyms for words. In this case, you would look up the word "hobnail" in a thesaurus and find its antonym, which is "smooth" or "plain."