Solder cast iron with lead free solder? Yes!

I had some holes to fill in a cast iron grinder base prior to
painting. I didn't have any bondo and I didn't want to heat the thing
almost red hot in order to weld up the holes without cracks. I did try
two different welding rods and silicon bronze rod but they cracked. I
expected it. I then heated up the casting pretty hot but it still
cracked. Not wanting to go through all the hassle of heating almost
red hot I decided to try lead free plumbing solder. It worked very
well. This is what I did:
Applied paste flux.
Heated from the underside until solder started to melt and tried to
flow but mainly balled up.
Used a brush loaded with flux to scrub the hot area.
This caused the solder to wet the scrubbed area.
I then used the typical techniques to fill in the holes. One hole was
about 3/8 wide and 5/8 long. After filling in the holes I noticed
looking at the back side that the solder didn't quite fill in the
holes so I repeated the flux scrubbing business on the back side of
the casting and got the wetting action again. I then filled in the
back of the holes. All in all the job went fast and the solder sticks
better than bondo. Paint sticks to it too.
I'm not entirely sure why the solder stuck with the scrubbing
business. I think that maybe the scrubbing action lifted the free
graphite particles away from the iron and allowed the solder to stick.
Even though the cast iron wetted it did not appear to be 100% wetted.
It sorta looked like it may have had really tiny unwetted spots. So
maybe about 90% total wetted area. I did try to pry the solder away
from the cast iron but it wasn't budging, it had really stuck. The
process reminded me of when I repaired a vise that I still use nearly
30 years since I made the repair. At that time I built a fire brick
enclosure around the pieces to be joined. I then used an oxy-acetylene
torch to first heat and then braze the cast iron. It took a lot of
flux and rubbing with the brazing rod to get the cast iron to wet. But
it did wet completely and then the process was straightforward. I
filled in the vee I had ground out where the vise had broken with the
brazing rod the way anybody would when filing in a space, like a large
fillet for example.
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Thanks for posting. Always good to know what works.
But I must say , I would have waited until I could get some Bondo.
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I really don't like bondo for through holes. But it does work and lots of folks use it with good results. The type with fiberglass mixed in works pretty good for through holes. I still like the metal solution best because I like metal. Eric
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I use to take fiberglass mat and pull fibers, small pieces out of it. Small scrap pieces work really well. Then mix them in with the resin to patch stuff like that. It kind of makes a snot ball that can be manipulated, formed pretty well.
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Leon Fisk
Polystyrene putty with chopped fibers in it works quite well both as a putty and as a glue for polystyrene parts. My dad's shop used to have the local Glidden supplier custom-mix such putty. It was excellent stuff. (May still be -- I don't know what the company uses now).
It won't necessarily stick well to metal, though. I'm not sure what body fillers like Bondo use to increase the bond.
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Tim Wescott
I found several youtube videos showing how to use acetone and "styrofoam" packing material to make some:
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I'll have to remember that. Usually have some of both around. Looks like it could be really useful stuff for making repairs...
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Leon Fisk
Blech. I said polystyrene, meant polyester.
Bondo is usually polyester, which is a catalyzed material and far superior to polystyrene. Bondo, for instance, should not melt if you dip it in acetone.
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Tim Wescott
Yeah, the original was just polyester resin and talc. They include some lig hter fillers now, and they have enhanced adhesion to steel, galvanized, etc .
Polyester resin in general is a very poor adhesive. Even in exterior home r epairs, repairing dents in siding and rotted window sills, there is a trend among the pros to use one or another of some lightweight epoxy resins toda y.
However, they typically cost at least twice as much; they have a narrower r ange of application temperatures; and they take much longer to fully harden .
I just used a couple of cans of Bondo to repair the 92-year-old clapboard s iding on my house. I was using it down to 40 deg. F, and it hardened in min utes. Used right, it's terrific stuff. Just don't count on it getting good adhesion in thin layers. It needs some mechanical grip.
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FWIW, that's one application that 3M (owners of Bondo) specifically recommend that it *not* be used for.
I don't doubt that it usually works, but resistance to gasoline for polyester falls apart as temperatures rise above 120 deg. F or so, and the gasoline resistance of styrene (a component of almost all consumer-grade polyester resins) is poor.
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