On 21/10/2020 23:21, Phil wrote:
On Thursday, October 22, 2020 at 9:39:13 AM UTC+13, wrote:
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?

https://www.weldingmania.com/mig/how-to-avoid-undercut/
Thanks! I've skipped through the article and think that's what I needed.
Now I know it's called "undercut" I can dig more deeply.