Filler

You could make up the difference with Milliput putty or  apoxie sculpt. Pretty easy to work with and long handling times.

Easier sanding than Squadron putty or Tamiya putty?

The beauty of that suggestion you've received is that those epoxy putties have a long working time and both smooth off with water. Apply a thin layer of the putty, then use a wetted finger to do most of your smoothing before it cures. Ideally, it allows you to come back later to use a bit of sandpaper to simply polish over the top to blend the repair in.  Trying to built up .75mm with either Squadron or Tamiya solvent putties is more difficult than it sounds because layers that thick require a heck of a lot more work. It often requires multiple layers of putty, which can distort or warp plastic if over-applied and normally requires several sanding sessions to get a smooth transition. Another advantage to the epoxy putty is that it can be tooled after curing, so you can rescribe panel lines with relative ease if you need to do so.

If the wing is two parts, top and bottom halves like most wings in model kits, when assembled at the root of the wing there is usually a gap.  Can you add a plastic wedge from scrap plastic or sprue to correct the issue?

I think I fixed it.  I used perfect plastic putty.  Interesting tid bit is even after it cures it's still water soluble as well as it's interesting I just ran some water over it and it actually fixed the issue.  

Peaches

I think I fixed it.  I used perfect plastic putty.  Interesting tid bit is even after it cures it's still water soluble as well as it's interesting I just ran some water over it and it actually fixed the issue.  

 

Just saw this post and was going to recommend PPP but I see that you discovered it. This product is great! One thing though, it will eventually start to get stiff and dry out but no biggie. Add a few drops of water into the tube and carefully shake and knead the plastic tube and will be good as new.

Depending on the particular case, you may not need to sand at all as with your wing issue simply by wiping most of the putty away with a water moistened paper towel or Q Tip.

As an example a gap along the wing to fuse join I simply spread the PPP over the gap then let it dry a few minutes. I then take a water moistened Q Tip and remove all the putty I don't need by simply wiping along the wing root, leaving a smooth clean putty line with no loss of surface detail due to sanding.

Interesting note, I bought apparently the new tube and they are aware of the issue and redesigned the tube so it doesn't dry out.  

I second Route 62. It's too late now and I didn't see the earlier post.

Adding putty like that risks later cracking if the wing is flexed.

A good thing to try next time: glue the top wing level with the root, and likewise the bottom. That moves the gap to the upper/ lower wing joint, which is easier to deal with.