In our last blog entry I discussed applying a guide coat to a freshly painted part, this time we'll discuss the process of sanding down this coat and you will see how useful the guide coat is.
I begin by sanding the primer with 400 grit wet sandpaper using a soft foam sanding block. The purpose of the sanding block is to create an even surface as you sand. If you sand with fingers alone over a large area, you will inevitably create ridges from the pressure points of your fingers. The foam pad prevents that.
As you sand away you will notice that the guide coat disappears completely from the high surfaces but remains within any pits or low surfaces. This is the function of the guide coat. It allows you to see two things: one, where you have already sanded and two, where pits exist requiring that you sand further. For the first primer coat I like to sand right down to the bare metal on any areas that have persistent pits. Once you have exposed the bare metal, if pits still exist then you will need a second coat of primer; this is normal.
After you are finished sanding the entire part you will typically need to apply a second coat of primer. It is important to scuff the primered surface prior to the second primer coat by using a scuffing pad. This operation removes any surface oxidation and opens up the primer to accept the next coat. Apply the second coat of primer the same way you applied the first, but this time you will notice there are fewer imperfections. Once the second coat of primer is cured, apply your guide coat.
I sand my second coat of primer with 600 grit wet sandpaper. This sanding session should go faster than the first because the surface is more prepared. You will typically find that if you did a good job of prepping the bare metal and sanding the first coat of primer, then the second coat of primer will be your last and you will not need to apply a third coat of primer. I find that some fans have such rough castings that they are more likely to require a third coat of primer for an utterly smooth finish, such as GE pancakes. This could be because my primer is thin or because I have not adequately prepared the surfaces with sanding, grinding and filler.
You will know the part is done and ready for your base coat of color when the entire guide coat has been sanded off of your last coat of primer and there are no heavy pits remaining and no bare metal showing. I feel it is okay if some bare metal shows on corners. Sometimes this is unavoidable. Just remember to scuff the final primer coat and clean it well with a surface cleaner prior to application of your base coat.
You can use this guide coat technique whether you are painting with high-end paints and primers or off the shelf rattle cans. Some of the details of this article may be different, but the overall rules are the same: the smoother the base, the better the paint job.
Figure 1: Sand paper wrapped around a soft foam sanding pad
Figure 2: Pits showing the guide coat contained within