Any cautions or comments, some fiberglass bottom work I had done is nice and white; someone suggested this stuff will do wonders but cautioned it's nasty stuff. Figured in the street, good gloves, respirator, wash down the street (good pitch) after. Thought I'd use some in the bilge too and maybe a few other spots.

Don't get it on your trailer. Bad reaction with the galvanizing.
I lay plastic over the areas of the trailer that might be exposed.

Aluminum trailer, but will take precautions, in hindsight keeping the shrink wrap would have been the smart thing to do; hindsight is always 20/20.

get the gel and put it on with a paintbrush

Bob, I have used it in the past. Works well, but hope you can hold your breath a long time as it is nasty. Respirator isn't a bad idea. Definitely gloves. And eye protection.
I now use FSR, or fiberglass stain remover. Usually cut it with water and put it in a spray bottle. I keep a bottle on the boat all summer to hit the waterline.

I wouldn't advise using the old spray bottle method. That stuff is incredibly corrosive. If it damages galvanized and aluminum surfaces, just imagine what it does to your lungs when you breathe the mist in. Apply with a brush.

To clarify, I use FSR in a spray bottle. Not On/Off.

It will also stain bronze thru hulls but man ohh man what a cleaning job that stuff does. Rust stains are gone in seconds. Hull stains. Hard water stains. All gone in seconds.
I poured some in a Tupperware (it's eats right thru paper paint cups) and then brushed it on. It's plenty thick to not drip everywhere. I was doing over my head without issues. Outside on a breezy day is best. You'll know when you get a whiff of it why a mask is a good idea.
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Great stuff. Just be careful with it. Gloves and mask is recommended. Put it on with a brush, preferably a long handled brush. Have plenty of water, or a hose handy.

Do I need to protect an aluminum trailer or hose it off as I go? I have a bottle so I'm stuck, sounds like long handled brush; glad I held on to the old boat brush.

protect the trailer. it will instantly react to it

The Works toilet bowl cleaner does work wonders as a hull cleaner
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The best way that I've found to apply it to a boat bottom while on a trailer is with a paint pad applicator with an extension arm.

I just bought some for general stain cleaner.

Good info..........