Weathering Pencils - what surface works best.

I want to use weathering pencils for the first time.  I have watched a number of videos and see ready to try to make dirt and rain streaks on the side of my tank.  But was wondering what type of surface do these work best on.

Currently my model has been painted using Tamyia colors then covered in Pledge to put on the decals. Originally I was going to do the oil color to weather it   But having these pencils, I think I want to use them. 

Should the surface be flat or gloss to work best with...