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Tom, if your galvanized trailer is a couple years old so that the finish is "weathered" there is a couple ways to paint the trailer. It is difficult to paint a brand new galvanized finish to make it adhere properly as Doc mentioned.
If you want to "professionally" powder coat or wet paint the trailer - the best option is to have the trailer sandblasted with a fine sand just enough to roughen up the surface. The paint shops will have a chemical pretreatment wash that will treat the metal before it is painted. The advantage of air dry primer and wet paint is that you will not have to remove all your axles and wiring before painting. If you go with powder coat you have to remove everything because the powder coat curing oven reaches 400 degrees F. for 15 - 20 minutes for a proper cure. The trailer has to hang on a dolly or conveyor to enter and exit the oven.
To paint yourself - just have the trailer sandblasted (or palm sander) first and then brush or roll on a good oil based rust paint right away. Don't wait too long after sandblasting or it will start to oxidize and the paint will not ahdere 100%. Most of the Home Hardwares still carry exterior rust type paints that are oil based ( like the old Tremaclad ) that you can have mixed in any colour. The new Tremclad paints are water based and are not as durable as the old stuff.

That's good info Doug! I always thought it was the galv. itself that was the issue (like painting chrome)..

The Galvanizing industry ( and Chrome) has had to remove the "final" stage of their process because of caustic health issues involved.
That "final" step coating was what gave galvanizing and Chrome finishes the long durability shine that we were used too. Since they don't have that good sealer finish anymore it is not as difficult to paint like it used to be. You don't see many newer pickup truck chrome bumpers that have that deep shine and will last 15 or 20 years like they used to.

and they rust quick too...interesting. Again, good info..

Thx Doug - very interesting. I've got a lot on the list right now to get ready for spring. But once the essential stuff is done. I might take a look at the situation.
Good to know it can be done.