I've been building a lot of 1/20 Tamiya F1 cars. Here's what I do.
1. Paint the wheel rim and fit the tyre to it.
2. Mask the wheel rim with a coin.
2. Brush paint Future onto the side walls
3. Apply the decals as per the instructions.
4. Brush paint Future onto the side walls again. This will seal it and remove any air pockets that cause it to 'silver'.
5. Spray acrylic flat varnish over the sidewalls until the desired flat finish is obtained. I find it helpful to add a minute amount of black acrylic to the flat varnish. it helps to tone down the whiteness of the decals a little. The way I do this is to dip the stirrer in the black prior to stirring the varnish.
Using this method, I've never had a problem with cracks appearing. However, it will crack if you try to fit the tyre onto a wheel rim after the process is finish. I think its due to the flexing of the rubber.
Cheers
Tony