classic car and truck

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How can I cut plastic model car pieces without ruining them?

I recently started building scale models of classic cars again for hobby/relaxation purposes. I had an idea to combine pieces from a couple different model kits to make sort of a “frakenstein” cutsom looking truck (a cab from one kit, a bed from another, etc) but a couple pieces, namely the truck bed, would need to be cut a little bit to make them fit together.

Does anyone have any suggestions on how I could do this?

For straight cuts it’s common to use a razor saw. The wide blade helps keeping the cut straight. Jeweller’s saws can be used for more complex cuts or difficult to reach places. The blade is very thin and can snap so be carefull with it.
There are also saw blades which can be mounted in hobby knifes. These can also be usefull in tight spots.

One more note about the dremel: I would not use it on plastic as it’s a bit of an over kill but if you do: use the circular saw blades, not the cutoff disks (like thin grinding stones). These create less heat and you have less risk the disc with shatter.

And just for completeness, there are also hotknives in the form of a U bow with heated metal wire or a knife mounted in a soldering iron. I personally wouldn’t use those either as the saws give more control.

One more edit to react on mjobien’s messge below: a filler is a very good suggestion. You will be needing quite a bit of it probably. The most common is the body filler with you can get in tubes in scale modelling stores. Just a creme to be smeared into gaps etc. Becomes rockhard be sanded afterwards.
If you need to build complex or curved shapes, then you can use epoxy putty (something like two component synthetic clay). Personally I would not recommend GS as it has quite a bit of memory and is harder to sand afterwards. I would go for a hardcuring putty like Milliput, A&B, Magic sculp, Aves, Brown stuff, or Tamiya. These can be sanded afterwards.

But I would not recommend green stuff (or kneadatite blue/yellow as it’s officially called) for this. GS was optimised for sculpting organic figures which have to survive vulcanisation. As such it has a fair bit of memory and stay a little rubbery afterward making it hard to sand. I would suggest a putty like A&B, aves, Magic sculp or Milliput. These putties can be softened and smoothed with water and will be sandable after curing. They are not as strong as GS, so if you want the best of both worlds go for 1:1 GS with one of the hard putties. Another filler is the body filler you can get in scale modelling stores.