The stuff I have from my boat is a pale yellow, almost liquid compound best applied by brush called 'Duralac' - quote:
'Duralac is a highly effective jointing compound which prevents electrolytic corrosion between dissimilar metals typically stainless fitting on an aluminium mast or boom. Ideal for mast fittings, winches, aerials, batteries, bolts, screws, rivets etc. Duralac is indispensable for the sealing of joints between dissimilar metals of all types, including magnesium and its alloys.
Used in the aerospace industry as an essential corrosion inhibiting material wherever dissimilar metals are joined.'