weathering decals

I usually use Humbrol Decalfix (other brands will do though) to really make the decal 'one' with the paint. This allows the rivets/panel lines underneath to show through. Then a light sanding, (I use micromesh, a very fine type of wet ' n' dry)
This just takes off the top surface and maybe a bit of the underlying detail pops through, don't overdo it though! After that, a light coat of matt or semigloss acrylic varnish to protect things. Don't forget, the decals that you put on, in real life are paint, and, as on the rest of your model, it chips and wears. If you can't get very fine rubbing down paper, try a pencil eraser (lightly) or maybe even toothpaste, it's abrasive and washes off with water ( it also polishes canopies!) Good Luck

Give me some information on what kind of acft it is.

At one time the services used decals for markings on their aircraft. Those star-n-bars, letters and numbers were in reality decals. They had a sticky coating on the back and after installing them they would seal them with a clear coating. Over time they would start to peel away, especially around high use areas. If they were installed over a panel that had to be removed it would result in exposing the screwheads and torn around the panel lines and screws. The decal would not loose its shine like the rest of the acft. They would start to peel away around the edges and at times whole sections would peel away.

After the services went to the toned down or black markings they would be painted on the acft. To simulate wear just use the base coat and dry brush over the markings. Not too much, you want to show fading not to make it too much like the base coat. Another way is to use a dull overcoat and put over the decal. That will take away some of the shine.

A little more information on the model and someone will be able to give you more specific information on how to tone them down.

Magor_au,
Berny13 and albertsponson are right on You could also use pastel chalk for weathering. So if you put too much on or do not like apperance you can simply wipe it off(this is done on a gloss finish).
To seal it go back over the areas w/ a flat coat(low psi).
Flaps up,Mike

Thanks guys for the info. i will go try it out .... a good excuse to start another model (not that i need one, lol)
magor