Hi, DD!
I would pretty much go with your description of melting down wheel weights to make ingots EXCEPT:
1. You NEED to flux the melt even at this point to evenly distribute the tin content. Tin is lighter, and having a melting point close to lead, will float to the top and produce that "skin" that you described. If you remove that skin, you throw out lots of tin! Flux can be almost anything that melts: candle wax, beeswax, old bullet lube, Crisco, etc.
2. Lead alloy is a mechanical mixture, not a chemical one. To keep the melt properly uniform from the time it is melted to the time it is poured into ingots, to be re-melted and poured into finished bullets, it NEEDS constant fluxing. Scraping the ladle or scoop against the sides and bottom of the melting pot will loosen impurities (ash, carbonized lubricant and oils, etc. and float it to the top for removal.
3. Now, of course, if your intent is to reduce tin content, then yes, skimming off the "skin" is acceptable.
4. Use a cast iron melting pot. Something with moderate surface area (to minimize "skin") and deep enough to contain a significant quantity of WW. I would avoid cast iron frying pans.
The above observations come from over 30 years of bullet casting and tutoring from a master machinist metal shop teacher whose skills included making cast bullets SO PURTY they bring tears to your eyes!
