I don't believe alloy wheels become 'porous' - for that to happen the metal would have to lose some molecular material to create voids. What does happen is surface corrosion at the bead, where pollutants (such as road salt) creep in. That has happened to all four wheels on my Pug 207 not long after fitting new tyres, making it necessary to deflate them, clean the beads and apply new sealant (or lubricant). If the fitter had done that in the first place I don't think the problem would have arisen - it hasn't since.
It can occur round the valve seat too.
Edited by Andrew-T on 20/01/2014 at 18:09
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