Got a 63 Chevy 1/2 ton and I bought some seam sealer in a tube form from Sherman Williams auto paint store. I have water leaking in from the roof of my truck during heavy rain. I removed all of the britle and broken up sealer that was up there for probably 47 years, sanded and applied this sealer that I purchased. Dont remember the name of it but about 2 days later I noticed that it had started cracking so its clear that it wasnt the right sealer or need something else to mix with it. Can anyone recommend a good seam sealer and the proper way to apply it.

http://img715.imageshack.us/g/trucks002.jpg/