Removing seam lines

Each piece of a resin kit usually has a seam line running in the middle from the two halves of the mold, unless the kit is very well cast. Getting rid of them is the least fun IMO.
Those ugly fingers aren't mine!

We're now working to get rid of the seam lines running on top of Misato's thigh. Shallow seam lines can be removed by using the back of your knife and scraping off the offending resin until both sides of the seam lines are leveled off. Scrap only in one direction, don't apply too much pressure.
But now you have the seam line area flat, and you need to restore the curve on the kit. Use sand paper smooth out the area. Start with 220 grit sand paper until you've smoothed out the area. Then use 400 grit to smooth the surface. It is safer to go up to 600 grit next if you want a even smoother surface, but I don't usually go that far. Sanding techniques were shown on the Removing Flash page.
For concave and hard to reach areas, fold the sand paper a few times more until you get an edge that you can hold on to. Sand the concave area with the edge.

Other techniques
If you kit isn't cast so well, you need some additional work.

For larger gaps, the piece will be deformed if you do scraping. In this case you fill the gap first before smoothing the surface out with sandpaper. If the gap is large, use polyester putty or SSP-HG. For smaller gaps, you can use Tamiya grey putty (or automotive glaze putty), Mr. Surfacer 500 or Zap-a-Gap. Each has different properties and you should use the one that suits you the best. Glaze putty takes a while to dry (>30 minutes) and shrinks some. But it is very easy to sand. Mr. Surfacer is slightly harder to sand than glaze putty and shrinks even more, but it fills the gap completely and will not leave air bubbles. Zap-a-Gap is the fastest to dry, but it can be difficult to sand.
After the putty dries, sand the surface smooth so that the seam line disappears and the curve in the original piece is restored. Becareful not to sand off too much putty. Due to the difference in resistance of materials, your sanding motion can get rid of more putty than resin, causing the seam line to resurface.
At this point you can prime the kit some and check if your seam line work is any good. I usually just check the surface reflection to see if the parts are smooth. If not, repeat the above. For larger gaps, it usually takes me 2-3 iterations to finish up.
Sometimes seam lines can be very hard to reach. I use a very low grit sand paper (120-150), fold it and reach in.

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