MIG welding question

One of my many projects is to restore the (large number of) 1930s steel,
Crittall, casements in my house, and the wooden frames they sit in. The
problems are that the windows have leaded lights and haven't been
maintained so many have been nibbled at the bottom by the rust mouse,
which lets-in the rot mouse. I've been getting the steel sand-blasted to
get the paint and rust off, cutting-out the rotten bits (Rage2 chop saw
- brilliant!) and welding-in new sections using a couple of spare
casements for sources of metal. The steel is a modified H section and
about 4mm thick so I'm bevelling the edges to about 1/2 thickness and
doing butt joints, either in-line or mitred at 45 degrees.
The problem is that the steel seems to slightly "run away" at the edges
of the weld, leaving a small pitt. When I look closely while welding it
seems to be generating small balls (not MIG spatter) but I'm fairly
certain it isn't galvanised. Any comments or advice from experienced MIG
welder wielders?
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Correction, I didn't realise they actually started making steel frames in the 1880s; Wikipedia suggests they started galvanising in 1939 (although I suppose they could have contracted that out).
I'm pretty sure mine are, but that's post WW2.
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My house was finished in the late 30s and TBH I thought galvanising started in the 40s. If they had been galvanised I don't think they would have rusted as badly as some have.
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Without seeing an image its difficult to diagnose but it sounds like you are either using too much heat or travelling in a straight line too fast. If galvanise were still present you would get lots of white smoke.
Try using a weaving motion erring towards the thicker metal.
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Yes, since Phil pointed me at the term "undercut" I've been educating myself and the problem seems to be caused by travelling too fast on too high a setting. When I tackle the next ones I'll slooow down and use a lower setting. I'm fairly sure it isn't galvanised because no popes have been appointed ;-)
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