So, slowly but surely, I've been gearing myself up for painting this little MR2 I've been rebuilding, and something that continues coming up is the subject of block sanding. I've already resigned myself to the fact that I'm going to block sand the entirety of the car, but I've got a couple of questions that seem to linger, and I'm hoping someone can help answer them so I can get moving on it Before anyone asks, yes - I AM using a guide coat (3m dry guide coat I believe), so that'll help keep things in check.

When I read about block sanding, I was under the impression that you primer your substrate with high build, let it dry/shrink, hit it with 180, and then step your way up by 100 grit at a time (going to 220, 320, 400, etc) to remove sand scratches from the previous paper grits. However, this seems to be in conflict with other things I've read - such as, I've seen people recommended to start their block sanding with 180 grit, and then REPRIME, and go straight to 400 or 600. To me, this doesn't make sense - as aren't you basically undoing all your sanding work when you shoot another layer on top of an already leveled surface?

Block sanding, while doesn't seem complicated, as the end result is just to have a flat/smooth surface to shoot paint on, it does seem like some people do this differently, and it seems like a stage where a lot can go wrong. Unfortunately, it's also been the stage where I've been the most confused. :/

Any clarification anyone can add, I'd be highly appreciative. Thanks!

i do all my body work finished in 150/180 prime and block 150/180 and reprime then block with 320 dry then use a orbital with soft interface pad in 320 sometimes 500 seal and shoot

after your first session of primer you should be flat, your guide coat will tell you. if you still see guide coat in some areas spot prime them and reblock with the 180.
when its all flat spray another coat of primer guide coat and sand with 600 or your favorite. you can also do as Dave does recoat and sand with 320 then the d/a. I'm old school and usually do it by hand with the 320-400 and then 500 or 600. and no the second coat or last coat of primer is not going to make your panels rough. the reason for the last coat of primer is that its tough to sand out 180 sanding profile with 600. that last coat really makes it easier.
Jay D.

First, remember this,
Coarse grits for getting it straight,
Fine grits to get it smooth.
Big difference.
If the panel is not damaged, no body work, and is flat and true, (straight)
then 400 to 600 over one spray session of primer is all that's needed.
Only when you're trying to get it straight and true do you need
the coarse grits, I use 150.
Using that coarse of a paper you'll probably break through
trying to rid the 150 grit scratches with a finer paper.
That's why most reprime and sand a final time.
Sometimes I can sand with 150, re guide coat it and wet sand
the scratches out with 600 without breaking through anywhere.
But you have to have enough primer on there to do that and
when there's that much primer it needs to cure longer before sanding to
fight shrinkage.

Ok, I think I understand a little better now. I've always been of the mindset ("always" being, since the beginning of my research ) that once you reprime, you're essentially starting the sanding process over again. I guess I was concerned that the spraying of additional primer on to the freshly flattened surface would be enough to throw a freshly flattened surface out of wack enough to need to "reflatten" it. It sounds like that is not the case.

Regardless, I don't think I have too much to worry about, as my panels are pretty dang flat. My issues with this whole process has just been that I'm constantly burning through when trying to make my way from 180-400 grit on the same primer layer as just too much primer is being removed over the course of using 3-4 different grits as I work my way up. The first pass with 180 is usually pretty good, without burning through in any areas, but as I work my way up through the finer grits to remove scratches, at some point I usually punch through, which was getting REALLY annoying. Sounds like the solution to this is to lay down my initial layer of high build, flatten with 180, let it dry, hit it again with primer, and then start with a finer grit to smooth it out.

Thank you very much for clarifying. I've had this car sitting in bare metal pieces for about 2 years, so it's time to wrap this project up. Sucks that the summer's almost done... I'm about to lose the temperature zone that I need to make paint cure properly at home. :/

my primer also mixes as a sealer and thats a option too, usually prime with a 1.6 but on my last prime i drop down to a 1.4 with 5%~10% reducer lays out pretty flat and easier to sand

I'm using KD3000, which thus far has been awesome. Sands just stupid easy, and once my gun was properly adjusted, went on silky smooth. I'll probably give a couple of panels a go tonight, and level them this weekend. I'm going 1.7 on the primer, 1.3/1.4 on the base/clearcoat when I get there.

colind wrote:
When I read about block sanding, I was under the impression that you primer your substrate with high build, let it dry/shrink, hit it with 180, and then step your way up by 100 grit at a time (going to 220, 320, 400, etc) to remove sand scratches from the previous paper grits. However, this seems to be in conflict with other things I've read - such as, I've seen people recommended to start their block sanding with 180 grit, and then REPRIME, and go straight to 400 or 600. To me, this doesn't make sense - as aren't you basically undoing all your sanding work when you shoot another layer on top of an already leveled surface?

stepping up grits is whats done when color/wet sanding clearcoat, but starting more like 1000( one thousand) grit.

colind wrote:Regardless, I don't think I have too much to worry about, as my panels are pretty dang flat. :/

Then there's no reason to sand with 180 grit.
Prime once then sand with 600 grit and paint.