One problem is, there were several types of tanks in use, some had prominent seams, others did not. If this is a seamless tank, then yes it can be a lot of work sanding the seam without destroying the detail, but it is something you learn from practice.
There is no good way to acquire skills like that other than doing the job enough times. One thought is until you gain experience, leave off tanks, ordnance, etc. Planes parked on the ramp are usually pretty bare.
Another key- get the fit as good as possible between parts before you apply glue/cement/solvent.