I had an exhaust gasket leak, so I replaced the manifold gaskets and used new copper nuts. however, it still sounds like there's a leak, so I'm wondering if I did an insufficient job of cleaning off the manifold, or if perhaps I need to replace the gasket between the header and the pipe.

If I take the manifold off, can I reuse the new gaskets that have been on there for one day? Can I reuse copper nuts?

thanks