I am going to get my scabby old MX5 inlet manifold and cam cover blast cleaned as part of my supercharger installation. The cam cover will be painted, probably crackle black but I want to keep the manifold as cast. What is the best treatment to avoid the dreaded 'white rust' ? I used WD40 on a marine outboard once and it worked surprisingly well, when thoroughly dried it created a matt glazed finish. I have also used ACF 50 but that seems to remain moist and attracts dirt. Frost have some 'cast finish' paint sprays but I am worried about flaking long term, so what is best?

You can buy a clear etch lacquer. That should stick OK. No idea how well it would last though.

I used High temp silver Aluminium paint
Looks like new and had mine on the car for 4 years without any degredation...Had a spare one which I did as well just in case, but never used it.
(Got the a spare painted inlet for sale if you are interested £10+ p&p) Looks like new

[Edited on 9/10/18 by Tazzzzman1]

There´s a product called ACF50 used in the aviation industry for that purpose.
Deep anodizing is also "rust" preventive as it is a finish comprised of aluminium oxide therefore protecting base metal from further corrosion.
HTH.