Wonky Plasticard

I'm doing a little model building work, some of which involves brickwork. My standard method for making 4mm scale brickwork involves breaking out a sheet of Mr Slaters most excellent embossed plasticard and sticking it on the surface requiring attention.

Mr Slater being a parsimonious cove, keeps the cost down to the hard-pressed modeller by using a cheap guillotine to chop the larger sheets into A4 sizes. This isn't a desperately accurate process as the cut lines aren't normally along a single brick course. I know this and always trim the sheet to provide an accurate bottom edge by running a blade along the grove of a course.

The next stage for this building involved producing a vertical edge at right angles to the bottom. Carefully sighting along the vertical brick courses is usually the way to do this. However, on this sheet, I discovered that the vertical courses aren't perpendicular to the horizontals.

You can see the line drawn by following courses compared to the "right" right angle set by the square.

To be sure, I checked a couple more sheets and they also weren't spot on, but a lot closer than this one. These were different bonds so from different batches.

How did this happen?

My guess, and it is only a guess, is that the plastic is heated up, pressed to emboss it and then cools down. I suspect that the cooling process isn't happening at the same rate over the entire sheet thus introducing the error.

Now, this can be a problem. I'm sure it's lead to some wobbly building work from me in the past when I've assumed that I could cut square by following the courses. Insetting windows and doors in a wall, it is essential that they follow the courses rather than the absolute accurate square lines so how do you "square" this?

For the moment at least I know. Future sheets will be checked. Iffy ones relegated to less well seen areas of a model. I'll also make sure I keep a reasonable stock of the stuff to hand. Don't want to pull out the last sheet on a Sunday and find it's a wobbly one!