I did a class for about 50-60 painters in Mississippi a month ago and that question came up about primer shrinkage.
Perhaps exaggerated a little I said you can apply 20 coats of primer and not get any shrinkage, if each coat is allowed to flash properly and then cure out good before blocking.
OR you can get extreme shrinkage with just three coats if proper flashing and spraying technique is not followed. Perhaps a little exaggerated but pretty close to the truth.
Myself, I finish filler with 80 grit and all metal finishing is with a 24 grit grinder and if in a good mood will run over the metal finish spot with an 80 DA.
Just my way done for years and will not change something that works.
Primer over paint I always use 180 on the paint.
*With that said, you will have no issue with how you prepped the car if you follow the following.
*Spray first coat wet, never medium or dry--you want to fill the scratch and not take a chance of bridging the scratch.
*Let each coat totally flash before applying next coat and same with third coat.
As pointed out, production shops will sand primer in 30-60 minutes and paint but if its a car you want to look good and last let the primer set for a day or week before sanding.
Because you can sand a primer in thirty minutes does not mean the primer is totally cured.
Although not touted (out loud) primers range from full cure in a day (rare) to 18-22 days being more the norm in this industry.
Nano technology is changing this but in this industry very few primers are using it at this point because of cost but a couple of companies are.
Last edited by BarryK; 05-13-2008 at 08:15 PM.