Its a boat trailer, covered in Zinc being the sacrfacial anode. You can definately try to prevent the rust by washing it down with fresh water after each trip. Some people apply lanolin or similar products.

In your case you can sand back the rust and apply a rich zinc two pack expoy like Internation Paints interzinc 52.

If you got an oxy tourch you can use a zinc stick.

Other option is accept your trailer is going to rust and look at completing an overhaul after 10 years.

 Thanks heaps for all the info, mate!  Will try sanding back and applying the zinc product.  Have heard that some people use fish oil?  Is that the same as applying zinc, or is it applied after painting the zinc onto the metal?

Fish oil is applied after the painting.

 Keep in mind thought it is an ongoing process and you need to spray every 6 weeks or so if the trailer is out in the weather. It won't kill any existing rust either.

 Thanks Michael

Phoenix make a product that is brilliant for protecting steel in corrosive enviroments. Google it , I think the place is in Oconner.

Has anyone gone through the process of getting their trailer sandblasted and then re galved?? was it worth the effort and what did you do about the rust on the inside of the box sections or did the acid bath at the galvanisers sort that out, and lastly does it last as long as the original galv dip??

 If you are regalvanising the galvanisers have a method to remove existing galv. Redipped just as new, I'll be doing mine in the next year or so. But as mentioned above you just need to flush properly with fresh water.