Hi Xenofon, do you have any local model shops that stock Tamiya paints? See if they have any Flat Base, you can use it to make a really good matt overcoat. Testor's ModerMaster range also has a good acrylic matt varnish that is very reliable from what I've read in the UK modelling mags.
You can apply either of these by brush, although it's better to airbrush for consistency on large surfaces of course.
I know an airbrush seems to be very expensive but they prove very useful in the long term for any modeller; they're one of those tools - once you have it you don't know how you lived without one! You can get a decent airbrush for very little these days (a cheap Aztek is only about 20-30 Euros, it's more than adequate for general spraying since it uses the same nozzles as the expensive ones) but the big problem is the air supply as any okay compressor is pretty pricey. I think the initial cost is well worth it for all the jobs only an airbrush can do, particularly if you work on any large-scale models.
Einion