try using 3M brand 5200 sealant, but only after you have cleaned the
rust,then apply, self-etching primer, then the 5200,

it's use [5200] is a watertight marine bonding/sealing agent, good
stuff...used to install items below the wather line, and above, hatches,
vents, etc. to keep 'em water tight............

                        no ya cant use it on heads, ...<sheepish grin>

richard