OK ,so far, we are 50/50 here. I called West Systems and talked to a tech person who gave the following advise. Apply 650 to new and old and let sit to tacky(45min). The strength of the 650 is that it will penetrate the wood better because it isn't as thick. Then once the 650 is tacky add a thickened coat of 650 with silica or add 655 and then put your scarf together. Seems time consuming but it would be the strongest bond West Sys. says.