Filling the small crevices and pits in a part is a crucial stage in creating that glass smooth finish many highly desire. Here are the steps and tools required to make your cast iron parts finger-slick prior to applying your primer of choice.
Tools Required:
- Polyester Glazing Putty
- Popsicle Sticks
- 80 Grit Sandpaper
- 150 Grit Sandpaper
- Piece of Cardboard or Wax Paper
- Foam Block Cut to Size
- Piece of a Bondo Spreader
- Lacquer thinner or wax and grease remover
Block sanding the bare metal
Step 1: Preliminary Sanding
At this stage, the part should already have been media blasted and smoothed out with a course Roloc Pad, sanding stone, or Carbide bits on a grinder which was previously discussed in Part 1. I prefer to only use the Roloc pads on surfaces that that are too high to be filled on “smooth” surfaces. It's important to clean off the part with lacquer thinner or grease and wax remover to ensure proper adhesion of the polyester resin. Start by cutting a small piece of 80 grit sandpaper and wrap it around your foam block pad. Using this coarse grit of sandpaper should be done with a foam block in hopes to avoid changing the round contour of the part. Fingertips can cause unwanted grooves in the piece. Now, sand in the area in which you will use the polyester filler. The part will be shiny and full of scratch marks when done.
Filler added to the bare metal part.
Step 2: Applying the Filler
Take a popsicle stick and thoroughly mix the polyester putty. It should have the consistency of cake frosting when done and be an even color. Next, knead the hardener until it is in liquid form. Apply a small amount or putty (about the size of a quarter) on your piece of cardboard and lay a very thin bead of hardener across the putty. Don’t add too much hardener!! This stuff dries rock hard in a matter of minutes! You will only decrease your working time by adding more hardener than is needed. Immediately after adding the hardener, take another popsicle stick and mix it well, ensuring all of the hardener and putty gets mixed together evenly. It should be a light blue color when finished. Take your bondo spreader and apply the polyester mixture to the problem areas. Press firmly on the spreader to fill all of the imperfections and ensure proper adhesion of the polyester mixture.
After sanding the greenish/blue filler will remain in the crevices creating a smooth finish.
Step 3: Contour Sanding
At this stage, you are waiting for the polyester mixture to cure. This will only take around 10 minutes or so and then you are ready to sand it smooth. Take your 150 grit sandpaper and foam block and sand the part until it is completely smooth. Don’t worry if the imperfections aren’t completely filled at this stage, we will address that later. Continue sanding the entire part until it is completely smooth and all of the polyester putty removed from the high spots. If there are areas of the part that aren’t completely filled, now is the time to refer back to step 2 and re-apply the filler. This may take three to four applications, but the more work you do now the more time you will save later during priming/sanding. You want the entire part filled and completely smooth BEFORE applying primer.