You can fill you iron with vinegar. set to steam after soaking for overnight, let it heat up, steam a minute or two over an old towel
I always steam, but press, do not iron!then dump out(carefully!) you might have to repeat several times--this always worked for me.
I use vinegar straight to remove starch build-up. wipe on, let sit, then wipe off....if it is stubborn, heat the iron on 1 or 2.