Card scrapers are an incredibly useful little tool, and are what people used before sandpaper was around. They are great for quickly smoothing down surfaces, and scraping old paint or rust off surfaces. I've used them removing stickers and varnish from tool handles so I can oil them.

These

Supplies

Stanley Knife blades
5 minute Epoxy glue
Some thin wood or plastic sheet (You can use this method to make some PVC sheets)
Sharpening stone, diamond plates or sand paper
Hacksaw
Clamps
Saw

Step 1: Cut Out the Handle

Cut out the handle from the PVC sheet or wood (this is the first time I've used PVC). I cut them into rectangles about 45mm wide by 50mm long. I used a small block plane to smooth out the saw marks, though you can also sand or file them out.

Step 2: Cut the Slot

Carefully cut a slot along the short edge of the handle about the depth of the hacksaw blade, use one of the blades to make sure any debris from the sawing are cleaned out.

Step 3: Prepare the Blades

Please be extra careful not to cut yourself during this step!

It may seem weird that to sharpen these into card scrappers you need to rub the blade around in circles over a stone or diamond plate (fine sandpaper can also be used). What you are actually doing is creating a burr on each side of the blade and it's the burr that actually does the cutting.

You should be able to feel the burr with your finger when it's ready.

Step 4: Glue the Blades Into the Handles

It's very important to clean the blades before gluing them into place as they are often covered in oil when new. Most cleaning agents will be able to remove the thin oil, but I like to use brake cleaner, acetone, or methylated spirits.

Mix up a small amount of epoxy and use something thin to push it into the gap in the handle, then coat both sides of the blade and insert it into place. Now clamp it and wait for the glue to set.

Step 5: Clean Up

Use another card scrapper or knife to remove any spilled glue. Lastly round off and smooth over the corners so the handle is comfortable to hold. You can use sandpaper if you want, but another scrapper is more fun.

You now have a scrapper ready to go, these can be sharpened over and over again.