Did you back purge? You will always get sugaring on a stainless weld if it isn't purged.
There are products on the market, i haven't used any of them. In my opinion on boost pipework i wouldn't take the risk. A turbo is worth more than a pile of scrap and starting again.
Back purging is to remove the air from the inside of a hollow section to be welded. By introducing argon typically to purge the air out. Sugaring is what happens when something hasn't been purged, the stainless oxidises inside the piece and grows into nodules inside. Only happens on things where the whole thickness of metal is melted. To the best of my knowledge.
Thanks for the replies. To be honest I didn't back purge it but I did set the amps so it didn't penertrate all the way through.
The inside of the pipe looks quite decent except for a small section were the pipe had a small gap and I had to add filler.
Unfortunately its not possible to get a file/grinder down there but I do have access to a shot blaster. My plan was soak it in a chemical then shot blast but I dont know whats best to use...