Just tried the AlumiWeld product from Harbor Freight last week for the first time, and was very happy with the results. I would suggest:
1. Clean oxidation, paint, and any surface dirt with a wire brush to ensure a good bonding surface.
2. Watch the manufacturer's product demonstration video (https://www.alumiweld.com/). It emphasizes the importance of heating the repair area enough to melt the rod on contact. You will never get proper adhesion by directly melting the rod and applying molten metal to a surface that is not hot enough to bond properly.
3. I discovered that a standard propane torch was not hot enough for an aluminum boat repair. While the demo video uses a propane torch to repair of aluminum tube, a boat will conduct the heat away so well that you cannot reach the required heat for the product to work properly. I purchased a Mapp gas torch at Home Depot and it was able to heat the area enough to properly use the product.
4. Use an angle grinder to remove excess and smooth the surface. The product is significantly harder than aluminum.
5. If you are unhappy with your first effort, simply reheat the area to melt the product and try again.
This stuff works great!