Thank you Timothy,about the spare tracks Icould't make a desicion about rusting them or not. But you are right, may be some drybrushing can do the job..
Von marshall,thank you very much. About hairspray technique the first and most important thing to say is: Don't use it too much,and don't apply it from a close distance! At my first trial I applied it from approximately 10-15 cm to the bottom of the tank and it removed all the paint! After correcting there,I applied it from 35-40 cm.and as a 'mist' (I hope it is the right word for this). The spray should 'land' smoothly on the model,it must not flow. When it dried,I applied white paint. And when the paint got dried,the fun part came:I washed the model under flowing water with a thoothbrush. I recommend a small one. This part was easier than I expected. But removing paint in small areas looks better,the only thing I would like to change is to remove the paint not as large areas but small and multiple areas.
As a conclusion this technique is effective andeasy,all it needs is a little bit attention and courage.
Hope we can see your successfull performance.