When we did ours, we used bar keeper's friend and some bleach. I see that Zep has a bunch of cleaners that folks might consider. Also, we used some sort of acid that is used on boats. It worked okay for the stains created by metal parts, but the bulk of our stains came out with good old elbow grease and a scratch pad. Or ten. We made sure to rinse it off as we went because it seemed like BKF would dry and kind of stick. So as he went our scrubbing the last time, I followed with the hose and a clean rag, wiping it off. I looked for "fish eyes" -- spots where the water seemed to run "around" an area. Those spots needed extra scrubbing.
By the time it dried, it was in the shade. Still in the 70s, so warm, but not in direct sun. We had two applicators, one which worked REALLY well and one that sucked. The one that I would use again was a big sponge wrapped in microfiber. The other was a sponge wrapped in a chamois. Microfiber is the way to go! If you can't find one over a sponge, just buy some cheap microfiber cloths from your dollar store. Note: I found ours in the automotive part of a Ace Hardware. It's neon green and I washed it out really well afterwards so we can reuse it next year.
My husband did most of the applying once I figured out my sponge was leaving runs in the stuff. He would dip the sponge all the way and ring it out COMPLETELY. Then, he'd apply it from top to bottom, all the way around the Scamp
. by the time he completed a circuit, it was dry enough to keep on going. He went around the trailer five times but the last time, he was just finishing off what we'd poured into the 5 gallon bucket. For reference, we only used about a third of the bottle, if that.
I had asked him to let me take some before pictures but his impatience won out and we didn't. He's kicking himself now, as one does when they are wrong.
We look at her every day and both marvel at how much DIFFERENT she looks. In a wonderful way. I'll try to get some nice "after" pictures in the sun tomorrow to show you the shine. Even at night, under the moon, she sparkles!