Alloy Wheel Flaking on the inside rim

[maverick54]

Had a slow leak in one of my tyres so nipped to the local tyre fitters to get it sorted.

 

It was leaking badly around the rim and when he pulled the tyre off he showed me the flaking on the inside of the rim and invited me to run my finger over it. I couldn't believe how rough and badly pitted it was.

 

I know alloy can't rust so what causes it to break down and flake on the inside of the rim when the tyre is supposed to have an air tight seal there to begin with?

 

He ran it on the machine and filed & wire brushed it smooth, then when he refitted the tyre and he brushed some pink paste like stuff around the rim and tyre. I'm guessing to help seal it over any imperfections in the rim. What was this?

Edited 3 March, 2019 by maverick54

[NZ100]

Hi M54, the pink stuff is to help the tyre fitter get the tyre on.  By the way, i used to enjoy watching MAVERICK, on tv.