Last para has an explanation of how I did it. Start by cleaning, I only used a light sanding block because the dirt was so ingrained which was also made worse by the light colour.
The previous owner was adamant that it was too far gone and needed a respray. Luckily (IMO) he sold it to me. The frame had a sort of ingrained dirt/discolouration, and was much worse than yours. I sanded off the dirt with a worn down sanding sponge and loads of GT85. With yours it looks like just using gt85/oil and gently sanding the rust patchs with fine wirewool would do it. Because most of your frame is white/cream, I think you'll be fine using something strong like jenolite in small doses to protect.
Then just touch up if necessary.
I'd recommend waxing it afte you've used tcut. it makes a big difference and helps to protect it. Just any normal 'wax-on/wax-off' style will do.
From another "don't respray" post of mine ;0
Last para has an explanation of how I did it. Start by cleaning, I only used a light sanding block because the dirt was so ingrained which was also made worse by the light colour.
Here is a bigger pic: http://www.lfgss.com/picture.php?albumid=698&pictureid=5464