Mixing Automotive Paint - Stirring The Pot

Primer Surfacer
Once the heavy lifting is done with the basic bodywork, after the bondo spreaders and fiberglass are put away, it is time to pick up the spray gun for the first primer coats. For metal cars, Planet Color's step-by-step refinishing guide outlines a system of etch primer, followed by a urethane surfacer. However, for a glass-bodied car such as a Corvette, there are different requirements. We went directly to Planet Color product specialist David Kidd, who directed us to their NP75 Ultra-Fill High Solids primer. This product bites like a bulldog to fiberglass or SMC panels, and is a high-build primer with excellent sanding characteristics.

For application over bare glass and body filler, the surface preparation consists of just cleaning with a surface cleaner such as Sherwin-Williams Ultra-clean solvent. We applied three even wet coats of the NP75 surfacer, allowing the solvents to flash between coats. Following the surfacer application, our Vette was block-sanded to perfection. Our initial block sanding is a hard hit with No. 180 on a long-board, which will give the maximum leveling effect for the block sanding effort. Next, any additional minor repairs are made, as required. After the first blocking, we will go back for a second primer application; this time for flawless surface texture.

Mixing
5 parts-NP75 primer
2 parts-US1-US4 reducer
1 part-NH77 catalyst

Mixing Notes
The pot life of the product once mixed is about one hour, so mix only what you will use. The US1-US4 reducers are all rated for different ambient temperatures, with US1 being the fastest evaporating, recommended for use at low temps; the US4 is the slowest recommended, for use at higher temps. We like to use somewhat slower than the recommended reducer, which helps to smooth the primer application as well as hard grainy edges. Naturally, a slower reducer increases the flash time.