How can I treat an aluminium frame ready for restoration?
There's a few approaches you could take...  

I'd say that getting it blasted should be the first step no matter what as chemical strippers can damage alloy frames are very messy and really bad for the environment.

Give it a polish with some very fine grit wet/dry paper to polish up the alloy.

Then you could use one or a combination of the methods below to get the kind of brushed look that you are after. Whichever gives the desired finish the best.

Tip Get a scrap piece of alloy to test brushing technique on before commiting to attacking your frame. (if you mess up badly you could always reblast or repolish the brush out of it.)

Option 1. (carefully) use a wire wheel drill bit go over your frame 

Option 2. by hand with a scotchbrite / scouring pad to go over your frame to get the desired brushed look

Option 3. 000 or 0000 grade Wire Wool then carefully do long straight strokes down the frame.

Once happy with your finish don't get fingerprints anywhere on the finish or they will show through the lacquer.

Clear lacquer the frame locking in the brushed finish. Thin laquer = no drips

Depending how shiny/durable you want the lacquer increase the amount of coats and let it completely dry. Polish with a clean microfiber cloth between each coat. repeat until happy.

Hope that helps.