Here are two methods that I like to use One is USC Icing it is available at any auto paint supply store. This is a catalyzed spot putty. It takes a little practice but the results are fantastic. The second method "the one I prefer" is take the left over sprue from the kit shave the plastic into very small pieces. Go buy IPS Weld-On 3 "available at any plastic supply store" Using a small amount of the weld on melt the Plastic into the mix until you get a consistency of Peanut butter. Keep it covered when not in use or it will dry out. Apply the Plastic putty in small amounts to the seams let it dry and you end up with a putty that matches the color of the kit and a putty that sands at the same rate as the plastic around it, as it is the same as the substrate. Hope this helps cheers John
1) Those ultra-fine grits you're using are no good for removing excess putty; they work much too slowly for that. They're for removing scratches, more like polishing than sanding.
2) Even if you sand away all of the putty, you'll probably see a colored blotch because the plastic will have been stained by the putty. Don't worry, it'll be hidden later when you paint it.
To remove excess putty, wet-sand it with 400 to 600 grit. If there's a lot of excess, use a file, razor knife, or metal scraper first to remove as much as you can, then use the sandpaper.
After the 600 grit, apply a coat of sandable primer and let it dry; this will hide the color of the putty and make it easier to see whether it needs any more work with the 600 grit. Then work your way up to progressively finer grades of sandpaper to gradually polish out the scratches.
=== Bill, My son-in-law is in the construction business and when I have a problem like yours, I borrow his high speed orbital sander with 150 grit sandpaper - works like a charm all the time.
Actually, I use denatured alcohol (DA) on a cotton swab or a cotton ball - first one I find. First soak the area where the putty is at with the DA and rub with the swab/ball. The DA will dissolve the putty slowly, so take your time. Let it air dry and check your progress. An additional assist is your trusty number 11 blade. Use it as a scrapper(spelling?). Place the blade about 80 degrees top part inclined towards you and drag it towards you - not too hard, just enough pressure to scrape just a bit off, do this several times, then go back to the DA, and repeat until you get the results you are looking for.