Another member wants to buy a RHS window from one of my parts cars. I got it out, cleaned it, and there seems to be a bit of a foggy section in the middle.

It's not a fog though, the glass is good.

I tried to razor blade it off, use alcohol, some RainX, but it doesn't want to budge.

Any suggestions?

If I paid $60 for a side window and it wasn't exactly perfect, I wouldn't feel good.

How do I fix this so I can feel comfortable selling it?

yes

white haze on tempered glass is a legacy effect of the original tempering (toughening) process.

it sometimes happens on architectural glass.
i remember reading about it years ago, its got something to do with the sheet of glass being near the start of the rollers or near the end.

anyway, its tempered glass on the side windows, so it sounds like you have got a couple of windows that might have been near the ends or limitations of tolerance during original manufacture.

you can google solutions to this, but i am unsure about how good solutions offered work on glass manufactured half a century ago.

google - white haze on tempered glass.
dishwashing liquid is offered as one possibility for cleaning.
see how you go.
but it does sound like its imbedded in the original material at point of manufacture.
if you clean it successfully after googling the correct search terms post up your success with it (or not) here.
its bound to affect one in every so many thousand of glass pieces.



its similar to the other effect of toughened glass.
ie spontaneous explosion of glass due to occlusions that form in the layer under the surface during the tempering process.
tempered glass is basically the result of heating then rapidly cooling the glass.
this puts the outer layer in tension with the inner layers of glass.
all sorts of things can form at the boundary between the two layers.
(when i say layers i am not talking laminated, i just mean boundaries inside one sheet of glass). i think the haze might form in the same way. its caused or forms at the boundary between the outer surface of the glass in tension and the deeper glass which goes into compression.
to get reliable toughened glass that will not spontaneously explode you have to destruction test it.

which is why apartment buildings are afflicted with glass balustrades to balconies that spontaneously explode. the chinese glass manufacturers won't destruction test it to ensure they erradicate the percentage of glass that is afflicted by faults that arise during the toughening process.

i think hazing is one of those faults. and it begins to show up over time as the glass ages.

there might be solutions.
but i am not sure. you can only google them and try them out.