I'm starting to scratchbuild several large (20"-42" long, 8"-10" deep) and was thinking about how I should cut these large sheets. Currently, I measure twice, cut once (really!) and do a bit of sanding to make sure parts are square and/or mate with adjoining pieces.
What I've been thinking about is getting a mat cutter. This tool is used by framers to cut mat board (the thick cardboard/paper that frequently is under and around framed photographs). While small ones, or cheap ones (which might be fine for infrequent use), will be around $50-$100; large ones designed for frequent use can be found in the $300+ range (too expensive for my needs). The Logan 301 family of mat cutters (used) is under $100 ($60-$80); still a bit expensive, but these seem like quality tools so will be worth investing in...and maybe I'll get extra use from them by taking and framing RR photos for the layout room. Be sure to use one with the 90 degree head, there is an approx 45 degree beveled head (50-55 degree I think), which might be a good start for mitered corners?
After I'm done with my half dozen structures, I could sell it online (or to another scratch-builder if it works well) and recoup some of the initial cost.
If anyone else has used this for cutting styrene sheet, please report your (the mat cutters') success/failure.
Using the back of an Xacto blade (with the sharp tip broken off), will gouge out a square shaped trough (you will see a curled bit of plastic being created as the blade removes, rather than dividing, the plastic (the sharp side of the blade pushes the plastic to either side of the cut line, which will form a burr on each side of the cut.
Looking down the groove, it will resemble a square trough, instead of a 'V' shaped trough with 'berms' at the top. Either way will work; I file (or sand by placing a sheet on a flat surface like a piece of glass, etc...) every edge using even, light, pressure before assembling walls, roofs, etc.
Small (.020) rod/strips can be glued into joints to fill small voids in joints, or use some kind of filler, then sand and paint. Sometimes white glue or artists gel medium (to thicken artists paint) might be able to be used to fill minor imperfections; I'm not sure how well either one sands.