I wash down with a solvent to remove the oily residue then soda blast.
You can make a soda blaster from a plastic soft drink bottle and an air duster.
Do you happen to have any pictures of that?
https://www.youtube.com/watch?v=qpkdaHxL7nAThis is as good as any other instruction video.
I make the hole in the bottle a little smaller than the air gun tube and warm the tube a bit before I force it through the bottle
After that I seal the area around the hole with some hot melt glue so the bottle is fixed above the gun as I found that it has a habit of rolling back down & it is a PIA to hold both the gun & the bottle in one hand.
Then when you want to let the trigger off turn it bottle side down because the bottle pressurizes a bit and you blow a lot of soda out when you turn the gun off.
The trick is finding a suitable bottle.
Again I run the hole a little different being just below the thread so I can do the cap up tight on the bottle and I drilled a round hole the same diameter as the hole in the tube.
Filing a square cornered hole is asking for the thing to split.
The gun I use is a bent one not strait as in the videos as I found it is easier to use if the nozel is pointing down while the bottle is dead vertical.
So look around for suitable bottles.
Long thick necked ones work better and keep them small , 500ml or less.
Keep the hole in the tube as close to the gun as you can because you get a stronger more concentrated blast that way.
Mostly i use it for cleaning small carbs inside & out before they go into the ultrasound.
The gunk comes off a lot faster that way & my cleaning solution lasts a lot longer.
Not real good on rust but excellent on zinc & aluminium.
With soda it is very important to have a good water trap on the air line or it will cake in the bottle.
The hardest thing is getting the soda in the bottle past the tube so again, having it pass through the bottle a bit further away from the end is a good idea.
Remind me in 11 days and I might take some pickies for you but right now I am packing the bags for the BSA national at Hahndorf SA.