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.
Eric