Well its been almost a decade since i made any models...
But i guess u mean weathering effect like dust and dirt and a lil rust ?

I used to do that using pastel chalks. Did great for my panzer models with diorama. Sweet stuff as long as u don't hold it too often. Details like real but wears off easily. Applied on "painted" models with a thin brush...usually those tiny tipped ones...and the process is looooong
After tat the model is basically a see-no-touch model
But i guess a top coat won't do any harm besides looking a lil fake lor