Watermark does not print correctly - pdf

Question

I'm using iTextSharp to add a watermark to existing PDF files. When viewing these files on screen, the watermark and PDF appears correctly. However, when printing on certain printers (2 out of 3 that have been tested by the client), the watermark seems to interfere with the content that is above it causing it to appear malformed when printed.
The PDF's are of CAD drawings (i.e. electrical circuit drawings etc.)
The code being used to apply the watermark is below. The PdfContentByte is retrieved by a call to GetOverContent from the class PdfStamper
private void AddWaterMark(PdfContentByte dc, string text, BaseFont font, float fontSize, float angle, BaseColor color, Rectangle realPageSize, Rectangle rect = null)
{
var gstate = new PdfGState { FillOpacity = 0.1f, StrokeOpacity = 0.3f };
dc.SaveState();
dc.SetGState(gstate);
dc.SetColorFill(color);
dc.BeginText();
dc.SetFontAndSize(font, fontSize);
var ps = rect ?? realPageSize; /*dc.PdfDocument.PageSize is not always correct*/
var x = (ps.Right + ps.Left) / 2;
var y = (ps.Bottom + ps.Top) / 2;
dc.ShowTextAligned(Element.ALIGN_CENTER, text, x, y, angle);
dc.EndText();
dc.RestoreState();
}
Here is an example of what it looks like on screen:
Here is what it looks like when printed on some of the printers:
Can anyone give me any idea on why certain printers will not print the PDF correctly and if I can change the way I apply the watermarks to avoid this issue?