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What you are seeing is a form of "filiform" corrosion. Use the search function, and you will find many threads about this, and how to proceed.
That corrosion is likely to be the least of your problems. The trailer is almost 20 years old, and if it has been stored outside in a wet, coastal environment, you likely also have rotten subfloor and a frame that is rusting away, especially if it has been parked in deep wet grass for any period of time.
Go to the top of this page, right under the big Air Forums logo, and click the Portal tab. Scroll down the page, keeping your eyes to the right had side, and you will come across both a utility to help you find a volunteer inspector in your area, but also, and more importantly, the inspector's checklist, which will tell you all the places to check and how to do it to see what condition the trailer really is in.
Good luck!
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