Don't use RTV, wrong all around.
I would not use any sealer, maybe a coat of silver aluminum spray paint if anything.
Preping the surfaces, clean down to bare metal, head checked for flatness, and above all, the cylinder liner to deck height (.002-.0045) above the block deck is very critical.
The studs need to be wire brushed free of rust, nuts tapped or replaced, threads oiled, torque to spec, then retorqued on first start up to temperature.
Do all this, it will seal.
