Wheels: the variations in weld bead success are mostly due to the diferences in line voltage to your machine, added to your technique. A small deviation in voltage becomes multiplied out of the machine, making the chart merely a suggestion for a starting point. By Your knowing experience, you get dialed in, by the results you see.
As far as etchants are concerned, it should be noted that hydrochloric acid is a gas, Hydrogen Chloride, with an affinity for COLD water. Created with chilled water of 40* it is saturated at 32%. Muratic is generally 12%, as bottled for stores for pools and common cleaning use. If you do some checking, you will find that toilet bowl and tub cleaniers are mostly Hydrochloric based, some approaching raw concentrations. Muratic is of course used in the household to remove Iron (rust) stains, that are complex iron and calcium deposits, and generally imune to soap based cleaners.
Sound familiar? Boiler wash compounds are normally hydrochloric, and become more aggressive as they pick up Iron as Ferric Chloride. When heated this action is accelerated. Another accelerant is Sodium Chloride. (table salt). Added to Muratic in small amounts, it is aggressive on mineral deposits bearing manganese and magnesium bearing calcates, depostied from boiler waters. This stuff is death to aluminum in nearly all alloys, and will turn near pure aluminum into a black fizzy with a pronounced exothermic reaction. Hidout is a problem with any salt or chemical action, so a neutralizing solution and thorough flush is always recommended. Just FYI.
Be Alert! The world needs more Lerts.