making aluminum solder adhere

Most aluminum solders, requiring no flux, are very exasperating in their customary refusal to stick. Usually this can largely be overcome by:

1. Very thoroughly scrape or wire brush the surface to be soldered. Carry this out to extremes and dig into each pit with a knifepoint or pick.

2.  Heat the aluminum piece to be soldered until the solder melts applied to it.

3. Keep direct flame away from the surface to be soldered.

4. When the solder melts, smear it over the desired area, and then play the flame directly on it.

5. Keeping the solder molten, scrape with a hook-sharpened bit of drill rod or a sharp file end. The molten solder will seal off the air while the scraper removes the instantaneous aluminum oxide, which is the source of the trouble.

While this method will facilitate filling pits, dents, scratches, casting imperfections, etc. in aluminum parts, the use of aluminum solder to hold parts together is not recommended since most of the joints made with it are weak and not reliable. For strength, welding is better. 

Antique Automobile
1947