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had my trailer re galved last year, when I got it back I sprayed the inside of all the cross members with lanacote then sealed the holes with flat squares of plastic and neutral cure sealer which will stop the rust coming from inside the cross members
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Have done the full strip down and regalv, was a right mission, fortunately trailer still had life in it, was a C section.
Went through this process again earlier in the year with my current boat (different from last) box section looking much like yours, I hummed and hahhed as either way was going to be expensive and /or time consuming fortunately sold the house and had some coin left over, got an alloy one made, so far has been fantastic, and a lighter load to tow.