Welding repairs, what to look out for?

[rollercoaster]

have borrowed a MIG welder and after a bit of practice on some similar metal to get set up,
I will be repairing this hole on my sill:



That is the rear of the passenger side sill, its got the sliding door over it.
What should I look out for, and any tips on how to go about it?
I will disconnect the earthstrap, but anything else too?
It has an ECU as its been converted to Subaru motor, any worries about frying that electrically?

Thanks

I am a proficient stick welder and usually like to gas weld things like this,
but probably only because I never got a MIG.

[jimpainter]

I disconnect the battery and also the ecu if it has one. Its just belt and braces approach.

[Plasticman]

gas it
mm

[rollercoaster]

There is something about recreating things like this with gas that makes it a pleasure,
can bash and bend things into shape much easier using a bit of cherry heat,
and end up with the metal in the same place as it was, not looking like a patch..
However there are disadvantages, like the paint and sealer is burnt off over large area,
and the whole thing seems to be very attractive to rust afterwards.
Also, fire risk is greater..
A good patch is probably a good repair?
As long as its sealed well afterwards..

I'll have a look at what is needed to disconnect the ECU.

[rollercoaster]

Ha Ha,
What to look out for...
more rust than you thought, thats what!
MOT man had a good poke about and showed me some areas to look out for,
also up top of inside front arches where the seat belt mounts are,
looked fine all covered in underseal, but the underseal flaked off easily revealing rust,
both sides will need work.