Starfury - First shot at weathering

The building instructions for the Starfury also contains a section on suggested weathering techniques. Bascially, you hold an index card against a panel line and lightly spray against it. Remove the card et voila!

I gave it a first shot (on the underside of the model), but the paint I chose (US grey) was way to dark in contrast to the basecoat (light grey). I kept working, just to get the hang of it. it was also the first time I tried the slightly finer nozzle (the black one) on my Aztek airbrush.

I'm going to go over it again with the basecoat and make the second attempt a lot more subtle, e.g. by mixing the colour I used in the first attempt 50/50 with the basecoat.

Just one problem : in order to have more control over the amount of colour you add to the model, it is suggested to heavily thin your paint. I did that, with a generous amount of the Vallejo thinner. Problem was that the paint was then too runny and created spiders on the model.
Do I need something else to thin the paint without making it run, add acrylic medium or something transparent like varnish or do I just need finer airbrush control? (suggestions welcome)