Welding Aluminum Using GTAW Aka the Gas Tungsten Arc Process - Compared to Welding Steel

Welding aluminum is harder than welding steel. Why? Because everything that can go wrong on steel is magnified times 10  when you are Tig welding aluminum.

Cleanliness, machine settings, arc length, welding with a dirty  tungsten electrode, torch angle, filler rod angle, type of electrode, and size of electrode . You name it, When you weld aluminum, its just more important to get all these things right,

If the aluminum you are welding has been left outside, like a boat dock ladder, or aluminum boat propeller, it is most likely heavily oxidized. It needs to be cleaned or it will weld like Fido's butt.
The machine settings have to be right too. Alternating current is the name of the game for tig welding aluminum and the  high frequency switch  needs to be on continuous...otherwise the arc will stutter.
Amperage is usually controlled with the foot pedal and with steel, you dont have to continually adjust amperage. At least not much. But since aluminum is such a good heat conductor, your foot pedal amperage control will get a workout. (Some tig welders even use the foot pedal to pulse the amperage to overcome heat buildup and to get the stack of dimes look.)

Arc length is important on steel, but on aluminum it is even more critical, too long an arc length and the heat will not be pinpointed enough casing your rod to ball up and contaminate, too short an arc length and metal will seem to jump on your electrode causing you to cuss like a sailor.
With steel, sometimes you can get away with a slightly dirty electrode... no big deal. But on aluminum, a dirty tungsten takes all the fun out of welding.  The weld will be sooty and will prompt your buddies to make fun of you.
Too much torch angle will cause the heat to melt the tip of your filler rod and the  filler wire will blob into the puddle,. This happens on steel too, just not as bad.
Type of tungsten electrode matters a lot too, especially if you are welding with a newer inverter type tig welder.  Old timers used to insist on using pure tungsten for tig welding aluminum. With the new tig inverters, using pure tungsten to weld aluminum will make you ask for a refund on your welding machine.
And what about the size of your tungsten electrode? With steel, it is possible to use a 3/32" tungsten for almost everything except razor blades. With aluminum, you need to use the right size electrode for different thickness metal. You really need to keep some 1/16", 3/32", and 1/8" tungsten handy and be willing to change electrodes when the  job calls for it...and sometimes even in the middle of doing the same job.

Do you get the picture yet? Welding aluminum is just plain harder because everything matters more.