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Discussion Starter · #1 ·
what is the real differance between sanding 400 grit wet and 600 grit
i've often heard go from 400 to 600 but i've seen jobs done with 320 and look fine. am i missing something,does the paint stick better?
could it be more control when sanding 600
just curious more than anything,,,,,maybe more the question what do you guys do?

i'm currently wet sanding with 400 on my 64 and was planning on going to 600 ,,am i wasting time or is there a differance
 

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In no way am I an expert here... but it seems to me that the more effort in the preparation, the better the end result will be. Personally, I don't think you are wasting your time... it is tedious but the result should be worth it.

'bear
 

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It partly depends on the color you will be spraying over the top. 400 is actually enough for everything if you use an epoxy sealer over your primer (which you should anyway) before you paint. Day in and day out I prep out all my primer with 400 wet for both metallic and solid colors. Where you get your difference is between solid and metallic colors. If you get primer shrinkage later on, which hopefully you won't but does happen, those 400 marks with become larger, and those 220 marks can show back through if you didn't block enough. 320 dry is nearly the equivalent of 400 wet since wet sanding cuts faster/doesn't load up as fast, but when you first start sanding with it, it WILL leave bigger, too big, of sand scratches. 400 wet is the minimum for pre paint sanding.

My opinion - if you will be spraying a solid color, 220 grit your primer down to straight. Let your primer set at least 2 weeks after 220 blocking (if you have the time to spare), to let it release all solvents and shrink back as far as it wants, then do a very thorough job of 400 wet. Epoxy seal and paint within the recommend recoat time for your epoxy.

If you are spraying a metallic color, use the same procedure as for solid colors, but then block with 600 wet. I will be spraying a very large metallic flake silver on my chevelle, and I can tell you that I will let my primer sit for as long as I can after being 220'd to avoid shrinking (after you 220, it breaks open the top layer and helps any solvents release), then 400 wet then 600 wet. This is just my opinion, I know there are guys here that say you should 600 everything even solid colors, but in my opinion it is unneccesary, since with collision jobs you can get away with 400 even with metallics if you do a thorough job. You could get away with it with your chevelle as well, but why chance it on such a big job?
 

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Everything I used to think I knew about sanding and paint adhesion is evidently invalid these days. Used to be there was the theory of mechanical and chemical adhesion. 320 grit was about as high as you wanted to go if you were using something other than lacquer that didn't bite into the previous coat. Epoxy sealer must stick so well chemically that it needs very little mechanical adhesion, what they used to call, "tooth". I'm sure you can reach the point of diminishing returns or else why not 800-1000 grit finish before paint. We use the 400 on solid and dark metallic, 600 on lighter metallic method.
 

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Discussion Starter · #6 ·
i agree more effort in the prep the better the job
i was more curious on the differance
never thought much about the primer shrinking but makes sense
thanks for the input
 

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If you are using a catalyst in your primer ( hopefully a high-build ) ther is no need to let it sit for 2 weeks unless you do not have a heated booth.In my shop we finish off all the body work with 180 grit,high build primer or spray polyester if needed,we then block the primer with 320,400,600 and 800 whether solid or metallic colors the smaller the scratches the less chance of problems.The reason people have shrinkage problem is the body work is finished off too coarse now you hope the primer fills in all the scratches,go the extra step the finer the scratch you leave the less material you need to fill it,the less you have to sand to polish it,etc.I use Glasurit in my shop a very high end paint line but the rule of thumb still goes for all the paint lines if you go the extra step it will pay off in the long run.As for metallic colors you should always finish off in 800 if your sand scratches are too coarse the metallics will not lay down correctly and will stand up and look rough and very coarse.A car I had completed over the summer scored a 100 points at Pebble Beach ( 1963 GT 250 Calif. Spyder) when it comes to painting my shop has won about every award but everyone has there own way,if it works leave it alone,if you tune it up with one more or less step thats fine also.
 

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My personal preference is 400 wet followed up by 800 wet. The only thing i would add is that i try to put the car out in direct sunlight for 2-3 days. No matter what i've tried with infrared or extended bake cycles to cure the primer the sun seems to shrink the primer best. For production work we generally use 400 on a da followed by a red scotchbrite and 800 on a da followed by a gray scuff on blend panels with out problem. Just because it works on collision work doe not mean i would use it on resto work. Also ppg's k36 sealer will work over the electro deposit plating that new parts have on them with out scuffing.
 

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I'm using Non-Stop sealer from Standox, it's actually warrantied to stick to OEM primer with NO sanding, just wax and grease remove and shoot. I still sand them though:D
I agree Mike, there's a lot of sealers being pushed now with recomendations that scuffing of ecoat isn't necessary. After seeing some of these jobs with collision damage and the large flakes of product coming off the unsanded ecoat I come to the conclusion that scuffing is well worth the extra effort. For the minimal amount of work scuffing the E-coat there's no reason not to do it IMO.
 

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Well i feel better hearing everybody else bag on gm e-coat cause i thought we just got the sloppy seconds out of the parts bin. I also sand the e-coat on the outer surfaces but on the underside of the hood well let's just say i have a lot less cuts and acrspes on my fingers these days.
 

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Well i feel better hearing everybody else bag on gm e-coat cause i thought we just got the sloppy seconds out of the parts bin. I also sand the e-coat on the outer surfaces but on the underside of the hood well let's just say i have a lot less cuts and acrspes on my fingers these days.
Gm will actually pay to sand them out, but you have to take pics and it's too much work for the office people:sad:
 
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