longitudinal rustoration, rust preservation in inaccessable locations Options

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I am currently repairing rust on my longitudinal/hell hole/jack receiver. There is surface rust in many inaccessible locations, between layers of sheet metal in the longs and spot welded seams and inside surfaces of the longs. I believe this is where the deterioration of our cars starts.

What products are being used to stop and prevent rust in these inaccessible locations? Are they effective?

Your comments and advice are appreciated in advance.

Thank You

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Sorry I should have included pics. here are a couple.

The rust is not that bad, yet, but I would like to stop it. I'm not going to probe any further there still is a lot metal thickness. I will replace what I have removed and do some further external reinforcement.

I once heard of a product that creped into cracks and crevasses by capillary action to kill rust but I can not recall where.

Thanks for your response.