Some Western model kit makers do not remove the  mold release agent before packaging. Kits under the Round 2 umbrella (AMT, MPC, etc) can be downright greasy at times. I have Star Trek kits that are pretty greasy. Moebius is the same in my experience. I built the B9 robot from Lost in Space and some areas in the cracks wouldn't hold paint and I didn't realize why at the time. At that time, I had mostly built Japanese model kits, so I was spoiled.

I prime everything (save for clear parts, of course). Use an actual primer and not just some other paint. Primers are there for adhesion. Go with a lacquer primer because lacquer is the sturdiest, like the Tamiya primer you have. I dunno where you  live, but I imagine Mr. Surfacer in rattle cans is hard to come by.

I don't bother wet sanding a primed surface, but for painted surfaces, it is best to wet sand. Only sand imperfections.

When it comes to cars and you are going for a gloss finish, don't worry if you have to wet sand even the final gloss coat. You can bring the shine back through using polishing compounds and later modeling wax if necessary.

I've heard that Testors Glosscote and Dullcote can turn yellow after a few years. I've had two people tell me this from their own experiences. One guy had a white Macross Valkyrie turn yellow and the other had a Millennium Falcon turn beige. YMMV, but I'd recommend Tamiya's clears.

I cannot vouch for Krylon or other rattle can products. It's not that I'm a snob, but I just didn't really build serious modeling experience until after I moved to Japan. The safest bet is to use paints that are made for the hobby instead of actual cars, fishing lours, etc.

I will say that it's best to avoid painting entire surfaces with enamels. Those are best used for detailing and such. Modelers here are baffled at how people in North America use enamels exclusively for their models. How do they use an enamel wash over a kit with an enamel paint job and an enamel clear coat? Beats me.