General Duralac anti-corrosion compound
HarryMann: Jointing compound - Corrosion prevention - bi-metallic corrosion
If you've ever removed the rear-hub from the radius-arm on a T25, you may have noticed a yellow or off-white dried-up paste that VW used between the two during assembly. This is a chromate jointing compound, to keep moisture out, separating the faces and preventing corrosion. It is particularly used when dissimilar metals are joined together, steel bolts in ally castings for instance, or large slightly uneven faces with voids brought into contact. Used a lot in the aircraft industry, Duralac is a trade name for the cheaper version normally used; a higher spec. for chromate compound is JC5A which doesn't dry and remains a paste indefinitely. Duralac (below) can be obtained fairly easily and cheaply, JC5A costs 5~10 times as much.
Duralac or JC5A is also ideal when jointing up timber and steel/ally, say when using bolts or rivets, or permanently planking a steel deck with ply.
Today, many would use Copper Grease, as that is more likely to be to hand and can work well for a while, but which can't fill such voids as well, between faces or bolts and their holes. Silcone and polyester sealants are also now available in a much wider range (e.g. RTV types), though 'stick or seal' is their main application.
Duralac: Inhibits electrolytic corrosion between dissimilar metals
Get Duralac (surprisingly cheap) or JC5A (Great stuff but surprisingly expensive) at aircraft sundry suppliers, : lasaero.com
Serious bi-metallic corrosion example - Steel mounting plate to end of T25 transaxle - slate chips were 1/8" thick
Duralac or JC5A would minimise this type of corrosion eating into the alloy transaxle case.