I had problems with an old mirror fogging up in the bathroom and tried an anti-mould bathroom cleaning gel. It kept the mirror fog-free for a few weeks and then I reapplied the gel as needed.

When I was a kid my parents ran a petrol (gas) station and one of the products was a little packet which contained a soaped up piece of cloth-paper (like the little hand wash towels you get at fast food places but steeped in soap) which you smeared on the inside of the windscreen to stop it fogging up. It worked really well.

I had a problem with a greasy windscreen inside my last car and my Dad said to try Mineral Turpentine. I wiped on a smear of that and it got ten times worse in an instant (what have I done??!!) and then as I kept wiping it away it cleared the greasiness and left it clear for quite a while.

Helen