When the two parts are to become one!In all my different modeling projects; railroad, armor and aircraft; whenever I am cutting and altering a plastic part to then be rejoined later I use the same process:
- First I cut carefully - always cutting overly long and then bringing the piece to its' finished length by filing and sanding. (Small moves Ellie, small moves.)
- Then I hold the two parts together dry and bring them up to the light to check that the two pieces mate uniformly. You should see a line of light of uniform thickness across the entire length of the parts to be joined.
- When joining the two pieces I coat BOTH mating edges of the plastic with a somewhat liberal coating of the solvent I am using at the time. Then I let those sit for a minute or so to soften the plastic on both sides of the union before doing anything else.
- Finally I push and hold the two pieces together forcefully, supported on a perfectly flat surface (a sheet of glass, or mirror or, in my case a piece of marble counter top.) I also, at the same time am holding one side of the piece against a fairly heavy metal straight edge. This usually guarantees a near perfect alignment, both vertically and horizontally, for this new union.
You want to use a bit of strong force here in pressing the pieces together. If done properly the two pieces of plastic will actually chemically fuse into one, single piece. Also if all goes well the plastic+glue combination will ooze out just slightly at the line of the union. Once dry this will mean you will only need to sand the new union smooth with sand paper wrapped around a sanding block. With a little bit of luck you won't need to use any body filler to make the line disappear before painting.***
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*** Don't waste money on an expensive sanding block. Any small solid, square, flat block will do. Wood, metal or plastic - it doesn't mater as long as it is flat and not flexible. The sanding blocks they sell in the stores will be too large for you model project anyway.
*** Allow this new union to set up and dry thoroughly - at least over night! With todays solvents you are creating a chemical reaction between the two pieces - it is NOT a glue as you might commonly understand it. The material needs time for the solvents to evaporate and the plasticizers to re-harden so you can sand it
*** Primer - you will need a good grade of spray primer. I recommend one of the Tamiya gray primers. (NOT the Tamiya white primer with which I have had much difficulty.) You will need to prime, let dry, sand and then repeat the process two to three times to finally create a union that won't be seen once painted.

I dare ya - find the seam in that repainted roof. Ya can't, cause I can't either when looking at the actual model - unless I turn it over and look at the unfinished bottom side!