5200 is recommended to be installed on metals using a primer first. 3M's material specifications recommend:
Scotch-WeldTM Structural Adhesive Primer EC-
1945 B/A
http://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSufSevTsZxtU48_eoYtvevUqevTSevTSevTSeSSSSSS--This is why I stick with epoxy based adhesives when making repairs.