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[Harvey Weidman]

I have been told that glass beading will close the pores, where as aluminum oxide will open the pores. When engine parts are coated (main, rod, brgs. cams etc.) aluminum oxide is used. This is how the coating impregnates the metal

In my experience I have not found that glass beading closes pores to be true. If you blast a smooth piece of aluminum with glass beads it will retain Gibbs. Compared to a smooth piece of aluminum that the oil wipes off. So the pores open when it is glass beaded. All media leaves residue and aluminum oxide is no exception. The residue may be what you are referring to.
The only way any sharp media blasting (of any kind) will close pores in my experience is when you use a smaller media than the roughness already on the part.
When you use smooth media like shot or smooth plastic, then the pores can close. If you use the right smooth media long enough it will actually polish the aluminum.
I tend to agree with Marco...Blast it then Gibbs it.

JMHO
H