Hi Robin, You have a perfect receipe for a new gap filling product… Probably a few million dollors worth of an idea… 
I did actually think about it. Lots of sprues on the kits in my stash that I could sell, say US $ 2 for a package with four 4 inch sections and a printed instruction sheet for Robins Seam Magic process.
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The nicest thing about the Miniart interior kits is that you get 20 or so Sprues. I’ve got stretched sprue for probably 30 projects from that one kit
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You’re killing my potential market 
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Erm right sorry about that!
I hear robin has special sprue, that is far superior to any other sprue available!
I already got to try this technique and it’s really nice! When I primed the gun on my archer build there was still a seam on the top of the gun breach, sanded it back, hit it with sprue and it’s good to go.
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Just put "AR-15 Productions - Robin’s Magic Plastic Filler" on the bottle and charge $12.95 for 10 ml or a third of an ounce and you won’t be able to keep it in stock.
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Robin’s very special sprue is from Shangri-la and dipped in holy water…thus filling in all your gap and hole needs.
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Alright so I’ve applied the stretch sprue method to some pesky seams on my archer build and after sanding they are GONE! Not a trace. I’m a convert to this method, fast, easy and seams stay gone.
Next up, how do you fill punch marks? I have the same problem I had with those mustang seams. I’ve been filling them with putty, wait a while sand them down till they look good only to prime over them and have them pop right back out. Any ideas here?
What putty?
Most model putty is absolute garbage.
Molak Stucco Putty, Tamiya Basic Putty & Red Automotive Putty are the only ones I’ve used that are allowed in the hobby room.
Depending on the punch mark depth and location.
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Shallow ones can often be scraped clean using a #15 scalpel blade.
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Thinned with Testor’s Liquid Cement, I’ve applied thin coats of Molak Stucco Putty or Tamiya Basic Putty. Needs to cure 24 to 36 hours. Sand and polish with tri grit sanding stick. Rinse & repeat as needed. Not suitable for metal finishes.
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Fill punch mark with super glue & apply accelerator. Sand and polish with tri grit sanding stick.
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Match punch mark to punch and die set. Punch disc from appropriate thickness plastic sheet. Use Testor’s Liquid Cement to melt disc. Allow to dry 24 hours and sand flush.
Typically for shallow #1 and touch up with #2 if #1 didn’t do the trick which is rare.
However for medium to deep always #3.
Rarely use #4 as exact match can be hard to accomplish but sometimes it’s a very useful trick.
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I assume you mean knock-out pin parks? Knock out (or ejector) pins are used in the injection molding process to apply a force to eject finished plastic parts from molds, and they often leave circular marks or depressions. This is especially true of older kits.
I deal with these using super glue; fill them with the liquid and sand the surfaces smooth when dry. I like super glue because it doesn’t shrink and dries quickly (with accelerator) but doesn’t get rock hard until after a few days. Great stuff.
Thanks @Armor_Buff and @TimReynaga those are the guys I am talking about. I have been filling them with mr surfacer 500, it can be made to work but often takes multiple fills since it shrinks. I’ll start using super glue on these.
How thick do you guys make your sprue goo? I dropped a bunch into an old jar of extra thin and was surprised how dissolved it had become after a couple of days.
Make it thick, its easier to thin it more than to thicken it up. Time wise.
I bought a punch and die set 30 years ago.
One selection of hex punches and one selection of circular ones.
One of the circular punches is a near perfect match for the most common ejector pins.
I punch out the number of little circles I need and fill in the holes.
Small differences in diameter can be handled with CA or sprue-goo.
Another method I use is to slice disks from sprue runners.
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Please show us some raw pictures.
After you’ve filled, puttied, shimmed, inserted disks, etc. You can still get an unpleasant surprise when you prime or paint. To avoid that frustration, I go over the seam / ejector pin mark with a red or orange alcohol based marker. Then sand with 600 or 800 grit. If you can still see the marker, you can still see the seam and have more work to do. If the marker is all sanded off, you have a perfect seam.
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That’s a great idea!