I haven't done a string quilt, but I have done a "crumb" quilt -- where I randomly sewed bits and pieces together. If I were to make a string quilt, I'd do it the same way. Basically all I did was heavily spray starch the pieces as I ironed. Made them oversized, spray starched and ironed, then trimmed to size. It worked really well. Not sure if it makes a difference, but I used Faultless Premium Professional spray starch (about $4 a can at Walmart). This starch never seems to flake on me, or gunk up my iron.