How to Use a Guide Coat - Part I

Creating a glass-like paint job seems to be a controversial subject amongst AFCA members. Some of us want a historically accurate restoration, complete with the bumps, lumps and pits of the original paint. Some of us prefer a restoration that is so detailed that the final effect of the paint is glass-like.  If you know me then you know that I am firmly a member of the latter camp. So without getting too political, here is how I do it:

The key to achieving a perfectly smooth paint job is to achieve a perfectly smooth base for your paint. We accomplish this using surface preparation and primer. For this article I am assuming that you already have the fan degreased and blasted down to bare metal: There should be no grease or paint remaining on the cast iron parts at all. After the fan is stripped you will need to prepare the metal by sanding, grinding and filling it to remove any casting imperfections.  Sanding can be performed with course grit Roloc pads on an angle grinder. Grinding is typically achieved by using carbide grinding tools on a straight grinder. In a crunch a Dremel with a variety of accessories will take care of much of this, although it may take more time.

After the surface is sanded and ground the next step is filling the imperfections. This step is very important as the better you fill the less time you will spend sanding down your primer and the less likely you will have to apply multiple primer coats. I use automotive grade polyester filler. This type of product will setup very quickly, you only have about one to two minutes to work it into the part before it begins hardening. I apply the filler using a small rubber squeegee and do multiple small applications to carefully cover all of the problem areas a few at a time. Once the filler hardens you will sand it down with 80 or 120g dry sandpaper until the pits or imperfections are smoothed over.

After surface preparation and filling comes primer. This article won’t focus on primer but let’s assume that you have a good primer, or at least one that you’re happy with. I have only ever used an inexpensive acrylic non-activated primer (Finish1 FP401 acrylic primer surfacer). The risk with a non-activated primer is that the primer is more likely to “fail,” resulting in it sinking into the scratches and imperfections that you will work so hard to eliminate during prep. So far I feel the fans that we restore are a good application for cheaper non-activated primers and I have not experienced primer failure. I attribute this to the fact that fans are kept indoors and away from UV exposure and excessive hot/cold cycles. I enjoy my primer because it’s inexpensive, dries quickly, and is very easy to sand. What I do not like is that it is not terribly thick so sometimes multiple coats are required to adequately fill surface imperfections. Again, I have zero experience with other primers so I can only speak for the experience that I do have!

After the first coat of primer is when you will apply the subject of this article, a “guide coat.” A guide coat is a light misting of paint or primer in a contrasting color above each coat of primer. I use rattle-can “sandable primer.”

In Figure 1 you can see that the guide coat (black) is misted over the primer coat (grey). The purpose of this coat will become obvious as you begin to sand the primer.

Next time we'll go over the process of sanding down this guide coat and you'll see exactly what function the guide coat serves. 

Figure 1: Guide coat sprayed upon freshly applied primer

Figure 1: Guide coat sprayed upon freshly applied primer