Hopefully we have some chemistry experts here..

I'm using Jasco Metal Etch to dissolve rust. It's phosphoric acid of unknown strength, and some sort of soapyness so it cleans too. The instructions say to dilute it with water. I've been working with the assumption that using it at full strength will give better/faster results. Am I right or am I just wasting metal etch? (It isn't THAT expensive, about $10 a gallon)

Jasco? I used to pour that stuff on non-diluted all day. Just don't get any on your hands... the stench is

I've tried both diluted and non-diluted. I found that without any water at all it didn't do much. After adding some water (less than they reccomend) it worked well. Maybe this was a fluke, but that was my experience.

-Tony

Hmm. One thing I have noticed is it only works when wet, and seems to dry pretty fast. I've started misting water on it after I've sprayed full strength on, and it seemed to help. That's what made me wonder if I should just start with it diluted.

Could be like Isopropylic Alcohol (rubbing alcohol), if it's 99% pure it won't be a disinfectant, if you change the mix 70/30 with water then it becomes a disinfectant. The acid may need the oxygen or hydrogen from the water to react and do it's job.