Knight,
What a lot of people like to do is when you've finished your paint job and your about to put your decals on, you give the model a coat of GLOSS varnish first. Then after you've applied the decals, you add your weathering and then finish things off with a coat of MATT varnish.
I can't remember the brand of varnishes I use, but there's loads to choose from in any model shop.