[p5b401]

If the aluminium has not yet been joined/wrapped yet then a coat of epoxy primer/paint is always a good start for the steel surfaces .
However, there will be movement between surfaces due to different expansion rates of metals which could affect a painted surface.
Galvanic corrosion protection tape such as 3M Scotchrap is probably the best and easiest solution.
If the joint is already in place and then the use of a penetrating metal sealer such as Rustmaster metal sealer can be used.
One other thing to watch is where dissimilar metals are joined by rivet, screw, bolt. Where these types of joints are required you can go through a lot of trouble with plastic washers and pastes but the easiest is to use anodized rivets, bolts etc. These will other a good first line of defense.