If your stretcher bars are affecting the appearance of your painting, first of all make sure your canvas is tight and try using a lighter touch. If you still have problems carefully slide a sheet of 200-300gsm card between the stretcher bars and the canvas. This helps prevent those unsightly lines you sometimes get when you press your brush into the canvas directly over the stretcher bars. The last lot of canvasses I bought have an additional stretcher bar across the centreline of the canvas (I use 18x24" size) and it's these ones I found I had the problem with. Using the card seems to soften the pressure applied against the edge of the bar. Remember to remove the card when you've finished!