[woodcote]

I used a grey primer. It will take some work to remove - or you can leave it on and paint over it.

There are also these 4-lane reed-switch sensors in two Tyco 15" straights (painted with grey primer)...

   

They work with almost all cars (including old T-Jets)... apart from some Micro Scalextric ones. The cable is wired to a 25-pin D-sub connector for use with Trackmate - although it will plug directly into a parallel printer port and work with software such as LapTimer 2000.