You wouldn't know it, since my Omega's a lovely shade of brown ATM, but I'm an obsessive car detailer... when I can be bothered.
Get to Tescos, and grab some of their own-brand shampoo, comes in big black bottles. Then go to the jetwash, and use only the shampoo and clean rinse modes. Don't use the brush...
Then get two buckets, use shampoo in one and clean water in the other. Go round the whole car with a wash mitt (about four quid from Tesco), using loads of water.
When mine's filthy I tend to use about four buckets of shampoo at this stage.
Get a hose or watering can, rinse the whole car down. Then use a "better" shampoo - Triplewax's is a good budget one, Meguiar's Gold Class is pricey, but worth it.
Again, use the two bucket method (this stops you dragging dirt over the rest of the car). Then get a clay bar (Meguiars' kit is about 15quid from Halfords, or Autobalm clay is about a tenner off the interweb).
If you get the Megs kit, it comes with Quick Detailer and instructions - if you get the Autobalm, you just need water. Spray whichever is appropriate over the panel, and move the clay bar over in straight lines, folding it in regularly.
Then give it another wash with the "good" shampoo, again with two buckets... rinse, then get a good drying towel (again, Halfords do the Megs one for about a tenner), use straight lines and barely any pressure.
Then get some AutoGlym Super Resin Polish, use a microfibre cloth or pad to apply, again in straight lines... and another to buff off.
Then get some Collinite 476 (about fifteen quid a tub) off the internet, if you get it with an application kit all the better. Do the same again, apply, let it haze, buff it off.
IF you want more of a shine, I can recommend Megs Gold Class, but the Collinite lasts for literally months. I am absolutely not exaggerating.
Check out
www.detailingworld.co.uk for more detail, but be warned, once you're in, it's hard to leave...
