I use a hack saw blade to widen the crack effectively cleaning the sides of the crack removing any loose material. I like to under cut the crack so that the base of the crack is wider than the top of the crack. I was afraid that a hairline crack might appear over time and the patch could fall out. I was over thinking the thing. it never occurred in 15 years. On a really bad banjo I bead blast the cracks and missing areas to get rid of the rust on the steel hoop so that the PC-7 will bond. I like to prepare the surface where the PC-7 will adhere just like sanding before you paint. It has worked for me.