Multiple times I've been asked how I created the additional armor on the raider above and other models in the past. What I have below is a step by step illustrated tutorial on the process.
In quick terms what you are doing is measuring the surface area of the model where extra armor is to be applied, creating a stencil, printing it out, gluing it to sheet plastic, cutting the sheet plastic out, then gluing the cut plastic to the model, then removing the stencil. Sounds more complicated than it is. Here's what to do.
Step 1: What you will need.
Spray Mount: Spray mount is an aerosol adhesive commonly used for photo mounting adn paper craft projects. If you never used it before, it's basically spray paint glue. This is what I use: 3M Sprayment
Plastic Weld: Plastic weld has many advantages over regualr model glue, the formost being that it's almost water consitancy allowing it to get into the tight recesses of a model joint. Also, unlike most adhesives, plastic weld does exactly what the name says; it "welds" the plastic together literally making once piece rather than 2 pieces held together by sticky glue. This is my personal favorite:
Blue Painters Masking Tape: This Tape is used in the template creation part of the process below.
Sheet Styrene/Plasticard: Available from most hobby stores. If you are doing large flat panels like tank armor, go for a thicker grade. If you are doing compound curves like the side of a Dark Eldar Raider, go thin. It'll help with bending the plastic during the gluing stage.
Software/Hardware: A digital drawing program like Adobe illustrator or comparable, a printer and scanner.
And lastly, Patience: This is a complex process that might take a few tries to get exactly what you want.
Step 2: After you decide on the area on your model where the extra armor is to be added, lay blue masking tape down across the surface. Smooth the tape so there is no bubbles, etc. From there you will be able to outline the area of the model that needs to be measured.
Step 3: After the tape is trimmed to the right areas, remove the tape from the model and lay it flat on a piece of white printer paper. This will serve as your base template for the stencil.
Step 4: Take the piece of printer paper with the blue tape stencil on it and scan it into your drawing program at 100% scale. From here you will be able to make a clean representation of your armor sketch.
Step 5: Using digital illustration software draw your custom armor pattern based on the measurements of the blue tape stencil.
Step 6: Print out the digital illustration of the new additional armor at 100% scale. Then using the spray mount spray the back side of the print out. Apply smoothly to the sheet styrene. Wait a few minuted for the spray mount to dry before moving to the next step.
Step 7: Cut out the excess areas of sheet styrene until you have a final shape for the new armor piece.
Step 8: Apply this new piece to the model using Plastic Weld. Apply the plastic weld to the non-paper side of the armor piece. If it's a larger piece you might benefit from putting the plastic weld in small cosmetics pump spray bottle to get an even distribution of the weld.
Step 9: Once the new plastic armor is fully adhered to the model, brush some water onto the printer paper. This will cause the spray mount to loosen up and the printer aper will slide off…and viola! You now have new armour on your model. Sand, gap fill and finish the armor as you see fit.
Here's a few pics of models that I have used this technique on.
Chaos Marine Baneblade
Now go cut some plastic!