On Thu, 27 May 2010 22:17:36 -0700, Tim Wescott
wrote:

Oops.

After finishing soldering my Very First model airplane fuel tank, I went
to do the last step of any soldering project -- cleaning off the flux.

Unfortunately, while I know that the flux I use yields to scrubbing with
hot soapy water, I don't know what -- if anything -- will just take it
off. I'm experimenting.

I've got some in each of four cups, with lacquer thinner, mineral
spirits, isopropyl alcohol, and plain drinking water. None of them seem
to be having any effect except the mineral spirits, which appears to be
separating it into two components.

In the mean time, I'm checking here to see if anyone has any pointers.
It's "Top Line Quality Soldering Paste Flux". Bought over thirty years
ago (I don't go through flux very fast). It looks like bearing grease,
& it sizzles when the iron touches it. Works great, but leaves a
residue which I don't want inside my engines!

So -- anyone happen to know if there's a common soldering flux that
contains calcium chloride and has this consistency, and if so how to
clean out the inside of an itty bitty fuel tank made with it? If all
else fails I'll run really hot water through it along with dishwashing
detergent -- but that doesn't leave me 100% confident.

You are removing zinc chloride and vaseline. Try brake cleaner to
cut the vaseline. If it's real heavy, use gasoline or perhaps
napththa for gross cleaning, then brake cleaner for the solvent
stage. Then I'd use a good proprietary alkalai metal cleaner as from
Birchwood Casey or Caswell, or make your own with TSP and sodium
carbonate or sodium hydroxide. Some add some sodium silicate but I
don't think it's necessary. These steps should result in metal that
is chemically and waterbreak clean, suitable for plating.

Since it's itty bitty, I'd at least try a run in my HF ultrasonic
cleaner with their pretty good cleaning stuff heated to 160F or so.
That doesn't always work but it gets 'er done surprisingly often.