Sagging happens when the paint is not viscous enough to hold itself together before it dries up. This defect is particularly common in liquid systems. If this defect is encountered, you may check the following list below where you might have gone wrong with your paint preparation or application.
Preparation: 1. Use only the correct diluents (either solvents, or water) as recommended by the paint manufacturer. Particularly speaking about OEM paint, I suggest that you only use the thinners that came with the paint. Should you use an alternative, seek proper advise to avoid paint defects.
2. Use the proper dilution ratio. Do not be any means overdilute paint. It will not only throw you off the viscosity recommended for spraying, the overdilution is also undoable. As good practice, add only half of what is the recommended by your supplier and check the viscosity in between dilutions. That way you will not overdilute your paint. Dilution ratios are a good guide to help you mix paint. Extra care should be observed if you are using a new paint system or color for the first time. Remember, we add diluents to paint to obtain the proper viscosity for application.
3. Check your expiry dates. Paints while kept unopened in original containers are good for a year, kept in controlled environments can be kept for two years. However, there are paint formulas that are stable enough that are still good even after several years provided that they are sheared with a mixer.
4. Do not overmix paint. Some paint thin (lose viscosity) while some thicken (become more viscous) after mixing with a powermixer. Be sure to follow the manufacturer’s recommendation for mixing time.
Application
1. Humidity. To achieve the best appearance from your paint (particularly for OEM paint), paint application should be performed in a paint booth, with exhaust and blowers to control not only the temperature but the humidity inside the booth. If there is too much humidity in the air, the diluent/s in the paint might not flash-off as expected leaving the paint wet for a longer time allowing more opportunity for the paint to sag. On the same note, if the flash-off is too fast, you might get solvent popping for bake-type paints (for cars, drums, parts, etc).
2. Wet paint thickness. Check your paint/air flowrates in the spray gun. You may also consider modifying your spray pattern if the paint applied on a part is uneven. If the paint is applied too thickly, the paint film will have a higher tendency to sag. The same goes for brushed or roller applied paint.
