What grit sandpaper or sticks should I be using?

Sandpaper is probably the most used tool in my shop. With plastic models, I usually don't go coarser than 320 grit and seldom finer than 600 grit. It's important to start with coarse, progress to fine by sanding out all the previous grits sanding marks. Once you get to 400 you're usually ready for primer. For clear canopies etc you might need to go to 1000 then polishing sticks. If you're still seeing a ridge, you just haven't sanded enough and if you can see the seam you haven't used enough filler. I often use the 4-grit nail files/polishers from the drug store - I think they work great. In re fillers, I like Tamiya or Bondo but Squadron green or white should work fine; super glue also works but is harder than the plastic when it cures and takes more effort for me. HTH

If you use superglue as an assembly glue and filler, which I do, try sanding after about an hour of applying it.  By that time, it should be hard enough to sand, but still not like concrete.  I've heard about using baking soda with it, but never tried it, since the viscosity of the gap-filling type seems good enough.

I've been using Zap-a-Gap gap filling CA instead of liquid or tube glues for several years, and used to let it dry overnight.  Now, I've discovered you don't have to wait that long.

The regular thin Zap superglue seems (to me) too thin to do much good, but the gap filling stuff (green label) is just right.

Like Bick, I don't usually go coarser than 320 grit, and for most builds, there's no need to go beyond 600 grit--unless you're polishing up to do a natural metal finish.

Good luck.

So tonight I tried wet sanding and am pleased with the results.  I used some sanding sticks I got at Hobby Lobby (skinny blue ones with only 2 grits, not sure what grit they are), followed that with my home made 400 grit sanding stick, and finished it off with the finest grit on a 4 grit nail fileing stick.  I've still got a little bit of gouging in some areas and a few slight ridges in others but it should suffice for my first attempt in so long.  It's a learning curve after all.  Thanx for the advice.

I don't use sanding sticks, merely wrap paper around blocks of wood or particle board. I keep several medium and small blocks on the workbench.  I typically use 320 grit for rough sanding if I did a decent job smoothing while applying  the putty.  I also use needle files a lot cleaning up putty.  I seldom use finer than the 320 grit until I am into priming surface.

If I read your post right, your problem is not so much the specific grit you're using, as the technique. It sounds to me like maybe you're just not removing enough material to get a nice even surface on both sides of your seams.

  The way I always think of it is: the coarser the sanding material, the more plastic you're removing with every swipe. Plan ahead and assess what grit you'll need for any particular sanding job. A succession of finer grits is just the "first aid" for the "demolition" you did with the coarser grits.

  As for myself, I use 220 for serious plastic removal, then proceed up to 320, then 600.  After 600 I can primer and not see too much difference in the surface.

Another way to remove a lot of plastic in a hurry with bad or offset seams is to use an X-acto blade as a scraper.  Hold the blade and 90 degrees to surface and pull the blade along the seam.  This does dull blades rapidly, so I keep one handle just for scraping. I put a used blade in it that is too dull for regular cutting, but will still scrape okay.  Do coarse removal with this, then go to sanding or filing.