Expert: William Forster - 1/6/2007

Question
I am a painter and I primed and sanded a car for painting but I did not get it painted for at least two months and I was told that it had to be resanded because it sait to long. I had finale sanded it in 600p sandpaper. the primer is a 2k primer from RM  

Answer
Hello Richard! How long after priming did you sand the car? My concern would be shrinkage, especially if it's a show vehicle or a collector car.  I would consider going back over it . Try this- you ever see that black carbon guide coat you rub on? It has a sponge applicator? Try rubbing a little of that on over a body work spot, wet sand with 600, and see if it shrank out. If it did, re-sand. If not, I would think that a quickie wet sand with a grey scotch brite is in order. After sitting for 2 months, it would do a nice job scouring off any contaminants, overspray, or dirt trapped in the sand scratches. If there is dirt  or overspray in the sanding grooves, it will affect adhesion, and possibly lead to fisheyes, peeling, or solvent trapping from the small dirt particles holding solvent from the base coat. You see, sanding a car makes paint stick because it increases the surface area of the painted part. A sand scratch is 3 dimensional- it has length, width, and depth. Anything stuck in the grooves, like a fine layer of dust or wet sand scuzz, partially fills the little canyon. An ounce of prevention is worth a pound of cure, Richard. I know, it's a bummer, and probably not what you wanted to hear, but it comes from the heart, and hard experience. Besides, if you have to wet sand the whole car again, it's not like you will be plaining down the orange peel again, you will just be removing any light shrinkage. That is where that carbon guide coat is nice. It's smooth as silk, and sands off like a dream- especially over previously sanded surfaces. If you re-spray guide coat with paint, you will have that grittiness to contend with, which is more time, and more work. I like easy. Bill