Yes, the chemical used to lower the PH level of your pool. That is the beauty of it too; it is readily accessible and it's cheap. A gallon will run you less than $10 for the good stuff and when I say good stuff, I mean the stuff you get from pool supply stores. Don't buy it from any of the big name stores, they let it sit around for too long and leave it in the sun to die. So what is so special about muriatic acid and how can it help you fabricate? Well for starters, it's pretty much the easiest way to remove rust. Just place a small rusty part in a cup of acid and walk away for a few minutes. When you return, the part will be shiny and rust-free. What I use it for mostly is to remove the zinc coating off of bolts and nuts before I weld to them. It keeps you from getting poisoned by the smoke that comes off of them while you weld and the bead is much cleaner too. As a bonus, it can also be used to ‘sharpen' files. Just leave the files immersed in some acid for a day and it will eat all of the built up metal and crap from in between the grooves and leave the file itself untouched. I will warn you though, the only real trick to remember when using muriatic acid on anything it to rinse everything thoroughly with fresh water and dry it immediately. A bucket of water to rinse your parts off is not enough. Remember, you use only capfuls of this stuff to treat an entire pool. The parts NEED to be rinsed with fresh water.
Why hasn't anyone told you about this magical elixir yet? Because it's a dangerous liquid! If you get even a drop on your skin, it will burn like hell immediately and if you leave a cup of it next to your frame while you work, it will rust everything the fumes touch, strip chrome off, severely irritate your nose and throat, and generally piss off everything around it. So be extra careful when using this stuff, if you don't, you will learn very quickly when you come out the next morning and everything in the garage is rusty and your hands are sloughing off the dead skin. The reason that I'm still sharing this information with you despite the dangers associated with it is because I believe that just as a chopsaw can cut off your hand if you don't take precautions, its usefulness is worth the effort of being careful.
Cold-rolled and P&O
Say what?! Anybody who welds knows how nasty the grey-scale that is on steel sheet is. It MIG-welds like crap and TIG-welds even worse. Try to grind it away and you either trash a good flap-wheel or gouge the hell out of the metal with a grinding disc, so just accept the evil scale and try to make the best of it. But wait, most thin sheetmetal doesn't have the nasty grey-scale. Why not and where does it come from? The grey scale comes from the forming process used to roll the steel into manageable sheets. “Hot-rolled” as it's called, is the process of heating the steel to a red-hot condition and then forcing it through a number of rollers until it reaches the desired thickness. Most of you have seen the nasty scale that rises to the surface of a red-hot piece of steel after welding or torching and can now understand where that nasty demon-spawn coating came from. Thinner sheets of steel (1/8-inch and smaller) can be produced by a "cold-rolling" process, which does not leave the nasty scale behind. The cold-rolling process also makes the material slightly stronger as well, so it's kind of a win-win. So what if you need material thicker than 1/8 inch? That's where the "P&O" part of the title comes from—it stands for "pickled and oiled." This process removes the offensive grey-scale and leaves a far more desirable welding surface behind on hot-rolled material. The most beautiful part of cold-rolled and P&O is that it is only marginally more expensive and totally worth the few extra dollars spent. You can thank me later.