Whoops! How would you fix this?

Flat base is not a paint in it's own right, it's an additive to be added in small quantities to normal paint to make it flat.

You could clean it up with some abrasive paper & give it another go. To get it flat some options are;

use flat paint,

use the semi gloss you have with a flat clear over the top of it,

use the semi gloss with a very small amount of flat base added - probably the easiest since you have both at hand. If you do this a good mixing ratio to start with would be 10 paint to 1 flat base. You would be well advised to do a test on something unimportant first though.

Thanks Milair,  I think I'll be going for option #3 tonight after some careful removal of the flat gunk left on there.

You have a couple of options.

If the base coat is enamel, you can simply remove the Flat Base using alcohol.

However, if the underlying paint is acrylic, the alcohol will remove that too. You may try applying a clear gloss (eg. Tamiya X-22 or Future) over the clouded areas. This will often restore clarity.

Just a word of advice from one of the biggest Lost in Space fans on the planet.  The robot's tracks were not shiny new rubber black after the pilot episode.  They were well worn and kind of a flat grayish black color.  Personally I would use a bit of ELO (Easy Lift Off) to lift onn the current coat (providing it's enamel), then redo it using a preshading in between the track grips.  Then use a mixture of MM flat black and flat dark gull gray.  Let that dry and then give it an acrylic wash with black to bring out the detail wear more.  Just a suggestion.

Sparrow,

Your observation about the tracks being brand new was exactly the sort of thing going through my mind as I wrote the OP, but my reference photo was a nice big clear pic of replicas on the assembly line.  I ultimately don't care that much about precise replication of anything when it comes to the tracks, I just thought the so-called semi-gloss paint was obnoxiously glossy and wanted to flatten them out a bit.

I carefully sanded the tracks back a bit and re-painted them with the same paint mixed with Tamiya flat.  It's not as bad as before but not awesome (it was a quick re-paint, after all), so I'll get back to it near the end of the project if I really feel like it.

Thanks very much for the feedback and advice, guys!