It's a tough job these days to keep up with the latest in automotive paint laws/restrictions, and consequently, use an ever-evolving product to coincide with these mandates. Most of us are in a semi-unique position with our project trucks in that we typically aren't going to be using the same products-at least under a coat of paint-as the collision repair industry, even though lots of research comes from that side of the painting game. In collision/insurance work on late-model vehicles, the repair shop will be doing its best to match what was done at the factory in both color and thickness, which is great; that's what they are supposed to be doing!
In the custom automotive paint world, there will most likely be a certain amount of sheetmetal repair/modification followed by layers of body filler, high-build primers, then sealer, and finally, who knows how many layers of single stage or base/clear paint that is a far cry from a factory finish-and that can be good, as long as you are using the right products!
With some custom paint jobs nearing 10 times the thickness of a current OEM job, using the right products is just as critical as the prep work underneath them to guarantee the proper finish and longevity. And before we get too far into primers, remember that what's done on top of each layer before final paint is only going to be as good as what's underneath. There is no miracle primer or paint that will fix shoddy and rushed metal/body/prep work; this also goes for those of you who plan on stopping at suede.
High-build primer is something that most of us have probably dealt with or heard about at some point in recent years, right? It too has evolved from the hard, resin-like polyester primer of the past into more flexible and stable epoxy- and urethane-based primers. Epoxy-based, high-build primers are great for direct-to-metal applications-they don't need to be preceded with an etching primer to treat the bare metal, and can usually be sprayed directly on steel, aluminum, stainless steel, and fiberglass, but aren't recommended for spraying on plastics. Using an epoxy primer like Planet Color/Sherwin-Williams' NP75 or Squeeg's Kustoms' epoxy primer basically saves two steps in the end over bare metal surfaces since you don't need a separate etching primer. You also get the first coat of high-build primer all laid down in one fell swoop. Epoxies generally have good sanding characteristics as well when it comes time to block 'em down. They also resist shrinkage while air-drying, which usually takes between 2-3 hours and up to about 24 hours depending on the brand, before being ready to sand. Besides not being good to use over plastics, about the only other drawback (if you can call it that) to an epoxy-based primer is that the colors it comes in are generally limited when an extra bit of color in the primer coat is needed for the finish paint color. For most of us, this isn't that big of a problem.
Urethane-based, high-build primers are a pretty cool breed of materials. They are far more flexible than the polyester high-builds and are made up of finer particles and lay out better on the surface. Poly primers were pretty chunky and rough in comparison. What this means is with urethane primer, laying out flatter, while still filling the low valleys, makes less work in the end to block it down since you don't have to sand the tops of all the texture off first, which is generally associated with poly primers. This aspect is a welcome one for those who have blocked out a truck from start to finish paint. Another neat aspect of urethane-based primers like the Planet Color/Sherwin-Williams Speed P30 SpectraPrime line is that it comes in seven colors including the usual gray and black. The other colors are surprisingly vivid: green, blue, red, white, and yellow. They can also be used alone, or mixed and matched for custom colors. With the P30 spraying out nice and flat, it actually has a bit of gloss to it, which is helpful as a guidecoat when it comes time to block it down.