Rust needs a few things to form. One of them is moisture/humidity. If you stop the moisture from getting into the panel/spot weld etc., then the chances the rust will keep going is going to be minimised if not completely eradicated. The reason a Datsun 1600 gets rust in the sills it for example is due to the sills not having paint on the inside, and water can get up in there and start the process. You notice the roof doesn't usually get rust on the top surface. The paint usually protects these areas if it is covered on both sides.
Seam sealer should be used OVER primer. Most seam sealers stick really well to metal, but stick better to paint. The other thing to be concerned of is if the sealer is caustic when curing, as this can cause rust too.
Good prep work, cutting out all rust and sealing the sections you can't with fish oil/POR products or a good quality zinc prime and paint will mean your Datto will last another 30 years...
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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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