Alumaweld works well the Harbor Freight stuff I only trust for cosmetic fills.
When working with Alumaweld stuff. You are really soldering with a zinc alloy. The part has to be absolutely clean and you have to use the flux that you can by with alumaweld. This raises the aluminum oxide(you can't see it) off the part. Don't use a steel brush to clean. If you have to use a brass brush. Small steel particles can get embedded in the aluminum and make it hard to repair.
I found on larger parts, it helps to "warm" the part over small burner like a Coleman stove at low temp to keep the part temp even so you don't get cold spots in your repair. Heat the part, not the rod. Keep you torch flame low enough on your part just to melt the rod. Too much heat causes scorching and you will have to clean again. Before running your bead, use the rod to "push" more of the oxide off. Then quickly go back and run the bead.
When you finish wash off any flux. The flux can attack the aluminum if left on long enough.
Aluminum by itself is a very reactive metal. It very quickly reacts with oxygen and creates a layer of aluminum oxide. It is so thin you can't see it. Aluminum oxide is so hard, it is the redish stuff that is on sandpaper. And not much likes to stick to it.
When you use a TIG welder the helium, argon or CO2 is what keeps the aluminum from reacting with the oxygen.
The flux for Alumaweld breaks down the aluminum oxide so you can get something to stick to the aluminum.
[This message has been edited by cmechmann (edited 10-14-2014).]