So do ordinary alloys, its called lacquer. A 4yr old factory alloy shouldnt "go porous". Whats with this reseating/gluing malarky? Sounds like your ham fisted tyre fitter has damaged your wheel. Go to a good tyre depot, Get it stripped down and inspect the rim/bead and valve seat thoroughly. Always fit a brand new valve (some cowboys refit old valves - which always leak) of the correct length (short for alloys)