Quote:To remove the grunge, use some engine cleaner as you have intended. Once that is off, then, as has already been suggested, use a good mag wheel cleaner & a stiff bristle brush.

You could also use a fine grade of steel wool that is heavily impregnated with soap. Place the rocker cover on a washtub with some nice warm water [about 1/3rd full] & see if you can brighten up the whole thing.

As for your 100Kpa WAP machine, I think it should be junked. 100 Kilopascals is only 14.5psi. Hell mate, two cups of coffee & a half hour wait, & I could pi$$ on it with more pressure than that.
I can guarantee that this pressure will not damage the alloy.

Once you have removed all of the dirt, & this might take a few applications of mag cleaner & steel wool, then have a go with a metal polish. There must be several brands around, but read the label to be sure that it is OK on aluminium.

Knock yourself out & it will come up better than new.

Aluminium must breathe???? It's not leather or red wine, it's aluminium, ...... a metal.
I think that you listen to too many ignorant people & do not apply enough of your own good sense. If it 'breathes' then it is porous. If it's porous, then it leaks oil through the material it is made from & that would defeat the very purpose of it's existence.

No my friend, the alloy rocker cover should not 'breathe' & a quality metal polish will impart a layer of protective wax to help seal the surface & prevent it from tarnishing & oxidising.