Here is the siege shield on my vindicator. I have had quite a few compliments on it, even from people that are far superior painters than I will ever be, so I guess it is done well.
Here is how I did it.
1) Clean and prime the plastic with gray spray primer.
2) Paint large flat surface with a thinned down vomit brown. It took three coats to get it solid.
3) Paint over with thinned golden yellow. Again around 3 coats to get good coverage.
4) Paint over with 2-3 coats of thinned sunburst yellow.
5) Wash entire surface with a thinned ogryn flesh wash. For a darker yellow use a thinned devlin mud wash.
6) Use a sharp pencil and draw your lines for the chevron pattern. I put little squiggly lines inside the sections that will be black.
7) Use a detail brush to trace your pencil lines with thinned chaos black. I went on the inside of the lines and worked out so I could keep the inevitable squiggle to a minimum.
8) Finish off the black with a thinned chaos black.
9) For battle damage I stippled boltgun metal over it. I kept this to a minimum. I also targeted any "squiggly lines" between the yellow and black.
10) I then stippled codex gray over it. Again, I kept this to a minimum.
11) Use badab black along the outer edges to add depth between the blade and the sides of the shield.
You can also use mithril silver and boltgun silver to add a bit of battle damage around the edges of the shield too with a bit of dry brushing and highlighting. I did it after I was finished with the yellow, but before I did the ogryn flesh wash. It gave the bare metal a rusty effect that I like.
Yes, it is time consuming to say the least. As a matter of fact, it took longer to do that the main body of the rhino. It was worth it I think.
What really made it look straight and even was the stippling. It helps to detract from the straight edges that are not so straight without covering up or detracting from the caution stripes. Hope that helps you a little.