I think you're over-complicating this:
Originally Posted by oldtiffie
The amount of mill scale really doesn't matter as long as you're using welding rod that will weld through it alright - in the States, 6011 or 6010 w/ DC are just fine.
The stub axle is close enough to concentric & straight.
To make sure the trailer tracks straight the important thing to keep square is the line across the trailer connecting the two front spring hangers with the long axis of the trailer. If the trailer is crooked, don't fix it in the axle - move the spring hangers. Make the axle straight.
If you're building this trailer from scratch, place the rear axle so that the proper 10-15% of the trailer load ends up on the hitch. This is the most important factor in insuring safety, since it prevents most instability problems.
Keeping the weld spatter off of the sealing surfaces IS important; I usually wrap the axle stub in heavy tape or similar during welding.
If you're worries about alignment, bolt up the hubs & wheels while stubs are just tacked in place and spin the axle w/ the wheels resting on the ground. Any out of alignment issue will show up as wheel wiggle. If you can't see/feel any problems, it's close enough to straight not to matter. Do remember that nothing is perfect in an analog world, but it's often good enough. Most commercial axles are straight; toe-in and camber really aren't needed.