Best grit for sanding primer

[SouthernMudSlinger]

Im blocking primer now and using 150 grit paper, and after i get done blocking, it seems to take a good bit of wet sanding with 500 grit to get the scratches out, and some are a little harder to get out. I spray 3 coats of primer on using the 4:1/2:1 (1/2 reducer for high build app.) I managed to get most out, but there are a few left here and there, but some times i start to go through the primer before i get them out. Can you go any finer than 180 and it still cut the primer down really flat? Let me hear the process of sanding ya'll go through before paint.

[Hannibal]

I am in same stage as you. I have a 2K urethane primer on. using 220 very lightly to level out valleys and peaks. Then go back with 400. On a door, I hold block horizontally while making "sweeps" at an upward (and downward)angle with the block kept horizontal.

I dont think I want smooth as glass finish on primer, just a slight drag when running hands over. You have to have some "tooth" for the paint to hold on to. The finer your primer is sanded (500+ grit) , the more likely the paint will "slide" off and cause runs.

I am by no means an expert. I have spent hundreds of hours researching and asking questions. This is my first paint job, so take it for what it is worth

[Roch_Greg]

Going from 180 to 500 wet has a big gap. I step through the grits so if I'm working with 180 I'll use 220 then 320/360 and stop there. That way I'm not working so hard on my final grit to get the previous scratch out.

But then I don't use anything coarser than 320/360 on primer (esp. the final coat) and even then very rarely.

[SouthernMudSlinger]

Going from 180 to 500 wet has a big gap

Thats what i was thinking, but we dont have anything inbetween right now, i'll have to get some. So whats the lowest grit I can go to and not show through the paint? We are going for a show car paint job.

[Len]

The grit you sand with will have little to do with runs if you allow your first coat to flash long enough before applying your second.

When you sand primer with 150 you should only jump about 100 grit up to about 240 then to 400 before applying top coats. Sanding primer with 150 is actually too coarse, you should sand it with 400 or 600 then apply your top coats. If you need to level the surface you can do it with 240 then jump to 400 but 150 is usually too coase unless you have some problems that should have been taken care of using polyester putty but now are being removed using primer.

[mmooney84]

Thats what I was thinking. I always. Start with 220 on primer unless it is real smooth then 400. Finish pollyester putty with 180. Finish Bondo with 80.

[SouthernMudSlinger]

Ok thanks ya'll. I went by the paint store and picked up a roll of 240 grit.

[isprayum]

some 3M dry guide coat will help you see if the scratch is gone