What I use for very small seams such as wing joints or tail surface joints is Testors plastic filler. It comes in the gray tube. I know, for bigger items it is practically worthless. But, for small seams it is very easy and does not require sanding.

Apply a small bead across the seam and then go over it again with your finger to push it into the seam. Then take a clean cloth dipped in alcohol, (the rag does not haved to be soaked) and wipe up the excess and leaving the pressed in filler in place. Give it a shot of primer and all should turn out well. No sanding. Only works with small seams.

Give it a try

I pretty much go with what the situation requires. I start with Tamiya thin glue, melt & squeeze to create a filler, sprue or plastruct for big gaps, super glue where I don't want shrinkage & Tamiya basic or fine putty for feathering. If I'm filling holes that go all the way through the part, armament holes in wings, etc. I try to use sprue from the same kit.

I do use Mr Surfacer but not too often. I find I need to keep applying it do to shrinkage when it dries. Any filler with a solvent reducer is going to shrink. Don't put a painted model with this type of filler in a food dehydrator to cure the paint. All your filling work will have probably shrunk as a result. Ask me how I know.

I've even use a soft plastic wood that comes in a tub. You can wet your finger & pull it over the plastic wood th smooth it. I use this in areas I don't want to lose detail.

Seems I use whatever is in my line of site to apply this stuff. I will use a exacto blade or toothpick for super glue & filler. My dentist gave me some of his used picks. There is one that has a small beveled end that is handy.

I know a lot of this is repetitive but I hope it helps.

I've used plastic wood that comes in a tub, Elmer's carpenter wood filler, though not over cardboard. It should work but I usually smooth it with a wet finger but you might want to sand it over cardboard. The water might deform the cardboard. You could also try sealing the cardboard with a light coat of Future before applying thecplastic wood.
Good luck.

For small thin seams and tiny sinkholes I like to use super glue. It can be sanded, polished, drilled and scribed just like plastic. Super glue sets in just a second with accelerator so there’s no waiting time at all unlike putty. Always make sure to sand right away, before the glue gets harder than the surrounding plastic. I use Squadron green or Tamiya putty for large gaps. They both have a fine grain and sand nice, but you have to wait several hours before you can resume work.

I also use gap-filling superglue whenever I need to fill a major seam on a model that must be sanded. The superglue has, in my opinion, two great advantages over anything else I've tried:

• It cures quickly (can be accelerated, too) and can be sanded directly after application
• Doesn't shrink at all - even the best of solvent-based putties keep shrinking for quite some time after application.
• Panel lines can be rescribed just as easily as they can be scribed in plastic.

For smaller seams, especially gaps along panel lines, Mr Surfaces is a more suitable choice.

/Martin

For small gaps I use superglue with baking soda as an accelerator.

Larger gaps I use a combination of plastic strip and Tamiya Putty.

For areas that require a bit of blending and you don't want to sand away detail I use homemade soft filler which is a mix of aircraft dope and body talc (mix to a paste about the consistency as Putty) and sand it with a medium and fine wet and dry sanding paper used dry. Like any filler though carefully mask the area to be filled as this will react with plastic and leave overnight. The filler dries pretty fast but the plastic underneath needs time to re-harden.