Welcome Rudi.
you have picked a great costume. Take your time and ask lots of questions.
for vapour smoothing, It looks like a neat technique but from what I have seen, you will still need to sand some. I have recently seen people using Bondo/Acetone wash that is painted onto the armour. It appears to do a great job smoothing out the print lines but will affect fine detailed areas. Thankfully, the death trooper does not have a lot of fine details except for around the muzzle.
I recently purchased an electric foot sander from Amazon. It is a small circular sanding disc and it makes sanding so much easier for larger areas.
I look forward to following your build.