Sand scratches show up as lines or marks in the paint film that mirror the marks in the surface being painted. They may also show up as streaks in the topcoat that magnify marks in the undercoat or substrate layer. These are caused by improper or incomplete final sanding of bodywork or primer coats (using too course of a paper), trying to cover scratches by filling ‘em with primer, or in some cases by sanding single stage or basecoat finishes before clearing them. You can fix ‘em by Letting the finish cure and then carefully re-sanding the area with an ultr-fine paper and then refinishing it. You can avoid sand scratches by graduating your sanding from course to fine papers, and not sanding basecoat colors before clearcoating (though if you do have to sand the basecoat for some reason make sure you apply additional basecoat color before clearing). Also use 1200-grit or finer paper for color sanding.
Grit, sometimes referred to as seediness, is the dispersion of solid particles of different sizes embedded in the paint surface. This usually happens when your paint material isn’t properly or completely stirred, or more commonly when you don’t strain your paint or primer. You may also run into a grit problem when using old paint (like that can of color you’ve had stashed waiting for the “right” truck to use it on) or by using material past its pot-life (the amount of time before the catalyst really begins to kick in and starts to harden the material). Repair options in the case of grit are the same as those from runs and sags—you can wipe the area with a solvent wetted rag and then clean and respray the area (seldom a first choice), or you can keep on going and wait till the paint fully cures and then sand and buff or sand and respray. Grit prevention can be can be achieved by mixing your materials thoroughly, straining all you under and top coats, and mixing up only enough material to use within its specified pot life.
Also known as edge mapping or bullseyes, shrinking is when a repaired area’s feather edging become visible shortly (within days) after a paint job is completed. Shrinking is primarily caused by shooting a topcoat before the undercoat has thoroughly dried. Or when you pile up multiple wet or heavy undercoats without sufficient flash time between ‘em, or possibly even applying a finish over body filler that’s not completely cured. This situation can be fixed by allowing the effected area to dry or cure fully and then sanding and refinishing as needed. You can oft times prevent the situation by making sure your body filler is completely cured before priming, making sure you thin/reduce your undercoats per label directions, and by applying undercoats in lighter coats and letting them flash to avoid bridging sand scratches.
Slow dry, or softness as it’s sometimes called is when your paint ends up dry but soft to the touch and susceptible to retaining finger prints or water spotting hours or days after it should be dry and hard. This can be caused by shooting your under- or topcoats really heavy and wet, not allowing sufficient flash time between coats, adding too much or too little hardener to your paint materials, or using the wrong thinner/reducer for your temperature conditions. You can avoid slow dry by making sure it’s at least 70-degrees where and when you spray, by using the correct hardners for your materials and mixing them in the proper ratios, and my using the correct reducers/thinners for the temperatures encountered. Rectifying soft film can be accomplished by force drying if possible, or by removing soft film and re-prepping and spraying.