Silver solder--what wattage soldering iron?

40 watts is MORE than enough. Are you heating the wire first, then adding the solder? Make sure to use alligator clips for heat sinks so as to not melt the dielectric. If your out of practice, practice on some spare wire of the same (or close to the same) gauge.

I do power cord soldering w/40 watts.

I would get a better iron. There are ones with variable power available now that I wish had been around when I could see well enough to use them. I also used Weller, usually 25 watts.

When I learned electronics in the 60's, acid flux was a no-no for electronics. It would cause corrosion in the weeks following the solder operation. Acid was for plumbing only.

I struggled with the same problem when I had a 40 watt Radio Shack soldering iron. A 25 watt Weller made an enormous difference. It seemed hotter despite a lower watt rating. If you plan to do more projects, get a better soldering iron. You'll save yourself a lot of time and cursing. I've heard excellent comments about this $15 station from Parts Express:

Stahl Tools Variable Temperature Soldering Station

Mingles, that one even looks like a Weller. For that price it would be hard to beat. It would be nice if it had a temp readout, but that would probably drive the cost up. :-) Heck, that would make a great backup iron.

Yep, I had one of those Rat Shack irons a while back. It is not the power rating that makes it not perform, it is just a cheap tool. I found it required longer contact time to heat anything up, then found that the soft material the tip is made from doesn't last very long and they don't sell replacement tips for it. I mean the tip literally burned away to a stub. In honesty, the poor thing was probably overworked with all of the projects I've done in the last few years.

With cartridge clips and fine wire you want something that will do the job fast so there is less chance of the heat traveling. Just my .02 based on my experiences.

I can highly recommend the Cardas silver solder. I bought it from Michael Percy Audio, but it can be found other places. The Cardas solder does seem to take a bit more heat, but once it begins to flow it works beautifully.