Thank you very much for the comments guys.
Albertas, no, I didn't find the cast iron that difficult to weld. Of course it is melting quickly, but there was no particular difficulties with the MIG, as the steel thickness of the elbow is much smaller than the cast manifold. And I haven't particularly prepared it except for the cleaning-grinding-degreasing part. Should I have ? For what reason ?
Anyways, after de-rusting the manifold by electrolysis (I just love this process, it is magic!)
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Voucht71, sur Flickr
... plug-welding the Scorpio sensor holes...
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Voucht71, sur Flickr
... some smooth black high temperature paint...
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Voucht71, sur Flickr
... it gives a nice finish.
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Voucht71, sur Flickr
Would be great if the whole engine was looking the same... may be one day !
Thanks.