Hi Judith. I test a lot of bag patterns, so maybe I can help. When the patterns I test call for heavyweight interfacing, it usually means something along the line of Pellon Decor Bond. If I am using a quilting weight cotton for the outside, I do like to fuse a woven to the fabric first, then fuse the Decor Bond to the piece but with the seam allowances removed to keep the seams from getting too thick. If I am using a canvas or duck fabric (not water resistant), then I will fuse Decor Bond directly to the fabric without seam allowances removed as I am not using a woven on this. Water resistant fabric may or may not allow for fusing. All of them are different, so you have to experiment. If you are not able to fuse on this type of fabric, you can use your interfacing by sewing it in at the seam allowances. Hope this helps.