Thing to remember with plasticard is the surface is shiny - very shiny. I use two methods, if its still flat not bonded together yet. I like to drag a stanley blade across it rather like a wood scraper to remove the shiny ness. If its already bonded into shape, say a wheelhouse, then I use 1000grit wet.dry paper to key the surface for paint. But always make sure there is no grease (fingers etc) before applying paint.