Adhesion & Cocktailing
It's a given that paint has to stick to the surface you're spraying it on, and when it doesn't, you've got problems, usually of the take-it-all-off-and-start-over type. "There are two ways that paint adheres," Prewitt explains. "One is physically; that's the reason you use different grit sandpapers. The other is chemically; the layers melt into each other. Acrylic lacquer won't take acrylic urethane clear. "It won't bond; it's like putting a piece of plastic on it." That's an extreme case, but if you start cocktailing back and forth with different materials, it's a crapshoot. And a lot of adhesion problems come from cocktailing, the mixing of different products from different manufacturers. The bottom line is that you need to stick to one manufacturer's product line throughout the job, ensuring compatibility. "I always recommend you follow the manufacturer recommendations, read the tech sheets, read the can. You know, most painters, the more experienced you get the less you think you have to read; I'm guilty of it."
Ironically, it's easier than ever to get this information-tech sheets are available online from both House of Kolor and PPG, among others. Prewitt boils this subject down nicely: "Between the chemical and the physical bonds, that's what really makes paint stick. You can't sand too smooth or it won't physically bond; you can't cocktail or it won't chemically bond. Knowing those two things can save a guy a lot of headaches."
If there's one constant in the bodywork universe, it's that primer and paint must be sanded with several different grades of sandpaper. It's essential to that smooth finish everyone wants, but it also helps the paint adhere to a body surface or undercoat. In this sample, we've got 36-grit (strictly for material removal), 80-grit for sanding body filler, 150-grit for block sanding primer surfacer, 400-grit for final primer wet-sanding, and 2,000-grit for color sanding. Scotch-Brite pads are handy for scuffing hard-to-reach areas. | 
One of the most critical steps in creating a smooth body surface is blocking. The primer surfacer is covered with a block guidecoat and sanded, revealing high and low spots in the panel. This wavy fender shows both-remnants of the guidecoat are visible on the left, indicating a low spot, and the bare metal on the right shows a high spot. The low spot will receive a coat of body filler and get sanded to bring it up to the level of the high spot before receiving another coat of primer surfacer. That coat is in turn sprayed with a guidecoat and sanded. This can be repeated as much as three or four times, depending on how straight you want your body. | 
Paint isn't good for you, so it's a good idea to wear gloves and even a respirator when mixing it. And when it comes to mixing paint, you have to use the right chemicals for the job, mixed in the right proportions to obtain the desired result, a good paint job that will last. It's never a good idea to cocktail chemicals from different manufacturers; if they're not compatible, you could be doing the job again. These inexpensive graduated mixing cups make it easy to pour out the proper amount of each chemical. |