Is 320 too coarse for sanding clear flat?

CK-2

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Reading the perfect paint job it says sand with 400-800 after applying first coats. Then apply additional coats of clear to cut and buff.

My question is would 320 wet be too coarse to use on the flat part of panels after the initial 3-4 coats of universal clear? Keep in mind this is fully cured clear that has been allowed to dry for several months. And it will all get 3-4 more coats after sanding.
 
In an earlier post, Barry said: I don't put information in the tech manual just for fun.
 
In an earlier post, Barry said: I don't put information in the tech manual just for fun.
I understand that. And I also understand that it’s hard to write a tech manual for every situation.
I would think that if you could sand universal that was a week old with 400, that 320 on six month old clear wouldn’t be that big of a deal.
I also understand that one person can use 400 to block clear flat and the next guy uses it and has burn throughs everywhere. That’s why I asked.
 
I get it. But, why risk it? Granted, tech manuals are conservatively written. Why not just get some 400 and see how it does? Why not start with 800? That may be good enough.
 
I had planned on using 400 with a hard block to sand flat and remove peel. But I have a roll of 320. Thought it might work to start with 320 on the flat places and also save some money on paper. I understand it’s not worth saving money to ruin something. But I also like to save money and effort if possible.
Just thought I’d ask and see what feedback I get. May be a crazy idea.
 
I have used 400 grit when doing a show car, it really gets rid of the urethane wave getting it dead straight.
I also tried 600 grit, it's very close and most can't see the difference, but I could.
I also had 5 coats of clear to work with.
There is a difference, you can see it in the reflections, like overhead power lines, whether they
appear as straight lines with crisp or blurred edges.
It's way over kill for most work. I get "plenty straight enough" for my normal collision repair jobs
starting with just 1500 on a random orbital sander.
The best straightest paint I have ever seen was by a painter (Bondoking on here) and he started with 320 grit.
 
I think it depends on how you shoot your clear. My 3 coats may be quite a bit heavier than your three coats.
Another thing would be the amount of peel that is present.
I view peel as little bumps on the surface. Sanding the clear flat is just removing the bumps to the flat surface without removing any more than that.
The coarser the grit the deeper the scratches and if the lowest levels of the clear coat are not thick enough you will cut through to the base very quickly.
 
My 2 cents. If you know how to sand then it shouldn't be a problem. But you would have to be careful. If you are fairly new at this and don't have a lot of experience you can make a mess real fast. I wouldn't touch it with 320 unless you had 4+ coats on it. 3 coats you would be far safer starting with 400-600. Oh and I'm assuming you are talking about 320 wet.
 
Sanding clear "flat" and/or straight is two different things.
The clear can be flat (no orange peel) but not even close to being "straight"
Two entirely different things.
You can get the clear flat easily with fine grit papers but you'll never
get it truly straight without using the coarser grits.
 
I have done it dry. I have no idea if it is recommended, but it has worked for me. 320 dry is my go to method for runs. I put 320 PSA on a piece of plexiglass and block them off with an "X" pattern. Keeping it dry works as the ultimate guide coat.
 
Lots of good info here. This is a show car finish I’m going for. I’ve sanded several things I’ve painted with 1500 and then 3000 and even 5000 with a orbital sander and buffed just to remove nibs and a little peel. This was all on daily driver repairs. If I wanted to just remove the peel I’d use 1500. But I want it straight.
I’ve taken great pains to block the primer on this car and get it as straight as I can. Used coarse enough grit to cut the primer, not polish it. I let this car set for a year in primer while working on other projects to thoroughly dry and do all the shrinking before my final blocking.
So now I’m at the clear stage and I want it cut flat and straight like my primer.

Some of these parts have been cleared for almost a year. I also understand how quickly you can ruin all that hard work. Ive done that too.
 
Ive sanded some with 320 dry, I had really slammed some clear on my fenders and had kind of a mess, 320 3m gold cut everything flat. If I remember correctly I did break through on one edge the next go around with 600.

In that specific situation I had 3 coats on, then sanded with 800, and then 3 more. At that point I wasnt happy so I got out the 320 dry and then the 600, touched up my edge and re-cleared. So most likely a fair amount more clear than you have.

Thats the one instance in my short time doing this that Ive done it. A few things I would recommend if you do end up sanding with 320, I would refine that sanding scratch one more step before you clear. So if you do use 320, I would use it dry as you can really see your progress, once you break the top skin of the clear and can see any unblocked spots are minimal you could switch to 600 and finish them flat and refine your scratch at the same time. That is kind of what I did, but I am by no means an expert, and I had a mess on my hands.
 
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Ive sanded some with 320 dry, I had really slammed some clear on my fenders and had kind of a mess, 320 3m gold cut everything flat. If I remember correctly I did break through on one edge the next go around with 600.

In that specific situation I had 3 coats on, then sanded with 800, and then 3 more. At that point I wasnt happy so I got out the 320 dry and then the 600, touched up my edge and re-cleared. So most likely a fair amount more clear than you have.

Thats the one instance in my short time doing this that Ive done it. A few things I would recommend if you do end up sanding with 320, I would refine that one more step before you clear. Another thing to consider if you do use 320, is when using it dry you can really see your progress, once you break the top skin of the clear and can see any unblocked spots are minimal you could switch to 600 and finish them flat and refine your scratch at the same time. That is kind of what I did, but I am by no means an expert.
By refining my 320 scratches do you mean block them out with say 400 before reclearing?
 
By refining my 320 scratches do you mean block them out with say 400 before reclearing?

Yes I would go out to 400 or 600 after 320. I personally would go to 600 and make that switch pretty quick if all you have on is 3 coats.

Keep in mind that its much better and easier to not burn through, you dont really want to touchup and or blend the base and have to deal with the issues that can present. Ive had one blend not go as planned and its not fun, a 40 dollar roll of sandpaper is cheap when compared to the nightmare that you could encounter if it goes bad by being too aggressive. Its a fine line between just right and disaster in my experience.
 
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Lots of good answers here, so let me just throw out a couple of things.

First will the clear cover 320 dry? Yes BUT IF you spray the first coat wet to fill and not a medium coat to bridge! Same as spray 2k primer bridging a scratch is the worse case problem that will show up down the road!

Second why? If doing three coats and sanding and three more coat why sand off most of the first three as we are going backwards.
800 wet will level about any orange peel or dirt you have.

Now if you really going nuts after done if you have lot of experience do first blocking with 400 or 600, if not 800 wet then 1200 and 1500
 
Will 600 cut the wave out and block it straight? I guess my question now is at what grit does it just become polishing? And how much difference does it make wet or dry?
 
800 wet will be overkill.
You also know you block clear front to back not like you do primer or you will make it worse.
 
So don’t block clear in an X pattern? Just straight lines front to back?

NEVER!! The calls i get had 3 or four coats of clear and looked great, sanded and buffed the clear and now its got urethane wave.
Think about this with let say 6 mils of clear and you do one extra stroke on an an x-block or a 45 degree stroke===can't be even.
 
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