I washed a small portion of my aluminum trailer with muriatic acid as a test spot (an inconspicous spot under the GN, by the tire).  It came out much cleaner.  I then did the lower 18" on one side (I wanted to do a small part at a time) and while for the most part it cleaned up nicely, I noticed that in the HAZ (heat affected zone) where the studs are welded to the wall, it turned black.  So now I have black lines down the side.  I tried additional acid to clean it, but it made it worse.  I tried a scrubbie pad - no effect.  I then tried a mild steel wool pad - big mistake.   Is it possible to remove this?  And how should I finish the rest of the trailer?  I do not want black lines down the side of my trailer where it is welded!!!!

See June 13 entry for a picture.  Maybe Retento can work his picture magic and post it here 

http://uniquehorsetrailers.blogspot.com/

"Gard" is usually the one that gives out some of the best advice regarding aluminum treatments on here because he has a lot of experience in the past at his job working with it.Not to knock any of Rentento's advice however.Just sayin.

But I will go so far as to say that I personally would never wash the exterior of any of my aluminum trailers with muriatic acid.They use that stuff to clean bricks with,after all.It is used to "etch" concrete floors and to halt corrosion on the aluminum flooring itself.There are several good and safe acid washes on the market,and barring the job involved using them,I'd take them to a truck wash.You want to clean it up,but you want to be kind to the finish.Especially with your investment: I understand you bought a 4Star? I have pictures that I don't know if I can post due to the pixels on the photo section of HTW,but,they were taken after I"d washed our 4Star with Streakmaster Aluminum brightener,and it did look very shiny and clean. 

I called local 'big rig' truck wash, and they had no clue either why it would turn black where the welds are located. 

Gard - any ideas?

I suspect that it is darker at the areas of welding, because welding reduces the heat treatment value of the base aluminum. This makes the metal "softer", changing its physical properties somewhat.

I don't know what caused the dark areas. I do know that there are many compounds that will enable you to polish the metal to a high gloss. Unfortunately, unless the surroundings areas are also polished, the specific areas will be quite visible. I've forgotten the name of the product, but recently a new company has been advertising its wares to clean and polish aluminum trailers in this forum. The owner indicated he was willing to discuss his products with any interested party. Perhaps he has a particular application that will spot clean the affected areas.

Sorry that I can't help

Muriatic acid can have a strong reaction when used on aluminum. Used full strength, it  stops corrosion, diluted it can be used as a cleaner. Because it is the same chemical as hydrochloric acid, but a weaker strength, (30%) it has the ability to etch many materials.

Most people use a brightener to (clean) their aluminum trailers. In effect, they are cleaning the surface and removing the oxidation, exposing the base raw metal. Many of these cleaners are fluoride or acid mixed materials. They are less aggressive than straight muriatic acid. If muriatic acid is used, dilute it with water to about a 20 to 1 mixture.

It may be possible to use a brightener on the black areas above the welding. I've talked to several of my mechanic friends, none of whom has encountered your results. We all think the area can be polished, but suggest trying to use another spot cleaner first.