I've used vapor blasting on engine cases and heads a few times, as Allan mentions above, they come out like new and the finish is durable since the process closes the alloy pores and stops dirt entering. Bead blasting opens up the pores and attracts dirt, so I wouldn't try that.
I believe hydro blasting is the same as vapor blasting, essentially its a mix of hot water, cleaning chemicals & media.
I've found that the finish with vapor blasting does eventually fade after 10 years or so, probably due to oxidization etc.
One option might be to paint the cases with high temp silver engine enamel which is what many Jap bikes use and is highly dirt resistant.
Last edited by gunner; 09/05/21 10:01 am.