How to get the Smoothest exterior finish?

Using Dremel 3D45
So far only PETG 1.7mm black and Dremel version of Cura Slicer.
Need strength also for light duty industrial applications.

your best bet is probably abs and use acetone smoothing.

polymaker also make a pla type filament that can be smoothd. But it's a more expensive solvent.

If the 3d45 isn't fully enclosed - it would be pretty simple to make it so and that's best for abs..
I know it's a replicator clone - just can't remember if it's fully enclosed and can't be bothered to go look :-)

Have a look at abs-x as well. Like abs but easier to print with and apparently no warping, so you don't have to design with shrinkage calculated.

vapor smoothing of ABS is fine but only gets so far like the flow out of brush applied paint. It depends on how smooth you want the surface. If you intend to make the surface as smooth as say injection molding or to have it metal plated for "say " perfume bottle caps, after vapor smoothing or even spray painting with a primer a process called print through will leave a little of the texture showing and mirror plating will make that horribly visible.

We came across this problem when one of our clients wanted to "gold" plate a batch of 3d printed replica FA Cups. The answer we came up with for was to use a 100% solids primer then sand the result.