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Probably get pre-cut masks - if available. Or take a deep breath and do it freehand - it's not as difficult as it sounds, although it might take more than one attempt. I normally paint the whole of the tyre wall very light grey (for the white wall), then edge the inside around the wheel rim, and the outer edge near the tread, in the tyre colour, using a fine brush and paint thinned just enough to allow an even, free flow.
If you can't find masks just take your time and don't get frustrated if you have to touch it up 3 of 4 times. I usually have to my self. I think painting wheels is my least favorite part to the model building process!!
What would be the bet way about masking a whitewall tire? For the Do...
http://www.ww2aircraft.net/forum/aviation/luftwaffes-whitewall-tires-19255.html
Found that thread!