BioKleen!

If you have to use a acid based product and don't want the hassle of covering the trailer you can use baking soda. Just dust the trailer with it. Baking soda will neutralize the acid on contact so no damage.

I'd be more concerned with any galvanized coated parts than the aluminum. That acid will eat through that stuff in no time.

www.slimygrimy.com

I used powerone on a buddys boat and the aluminum trailer had spots where I sprayed it. I did use a garden sprayer to spray it though. I did my boat this past weekend and I just put powerone on a rag and rolled around underneath my boat on a creeper so nothing got on the trailer and did the whole bottom in 10 minutes. Powerone works great with no bad smells or having to worry about it getting on your skin.

I have been using On/Off for years with my painted trailer and I just wet the trailer and then hose it off well after I clean the boat. Never had an issue.

Power One is great for the boat, but just kills my galvanized trailers and diamond plate.

Wet trailer makes power 1 do more damage, fyi. Power 1 works great though. Best rust stain cleaner I have ever used. I just did my bottom with FSR on a rag, not a drop on the trailer. Very quick too.