Max Grit of Sandpaper to Use Before Priming?

[Bob B.]

Hi all,

I'm getting ready to prime my '57 body and am going to use high-build primer. What is the maximum grit of sandpaper to sand the body that won't show the sanding marks after priming. I'm thinking it could be pretty coarse.

Thanks,

Bob

[Domenic T.]

Re: Max Grit of Sandpaper to Use Before Priming?

Bob,
lots of answers on this one but depends a bit on what kind of primer and how good your body work is. Also is it bare glass or over another paint?
The first primer I use 220 wet 3M paper. Then apply a second coat of primer and I use 400 wet. I usually change the color a bit on the second coat to make sure I keep it just above the first coat. @ part primers are the best in my opinion. I have a paint toner I use to tint the 2nd coat of primer. I am sure the factory only used 1 coat but in the shops 2 or more are usually used. The total primer build up should be thin when all sanding is done. The old laquer primer (when thick under paint) was soft and stone chips would happen easily. They called it the egg shell effect.
There are many good painters on the forum and I think you will get a few other good opinions.

Dom

[Bill W.]

Re: Max Grit of Sandpaper to Use Before Priming?

All primer & paint shrinks with time and as it dries. Why try to cover deep scratches with primer ? I finish all body work 220 and my bare fiberglass with 220 / 400 . The max should be 80 . The least amount of material you put on the car the longer it will last . Less is best .

[Douglas L.]

Re: Max Grit of Sandpaper to Use Before Priming?

I've found that most poly primers will cover 80 grit marks and thats what I usually sand to in the beginning. Once I get to the point that I'm pretty sure I'm going to apply my last coat of poly primer I'll sand to 150 or so before that coat.

[Mark E.]

Re: Max Grit of Sandpaper to Use Before Priming?

Reference the tech sheet for your primer-surfacer. That usually provides surface preparation guidelines. Some stuff builds and fills better than others.

Also, for a given grit of paper, hand sanding will leave deeper scratches than a dual-action sander. The coarsest paper I'll typically use before the first coats of a high build primer-surfacer is 180 by hand or 80 by DA.