What grit to use?

General Discussion. Make yourself at home...read, ask and answer!
Should this post get stickied?

Yes
582
92%
No
22
3%
I don't care, I already know how to sand.
28
4%
 
Total votes : 632


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PostPosted: Thu Nov 25, 2004 8:28 pm
Stripping: 36-80
For metal I use 40, fiberglass or plastic 80 might be called for, 36 grit scratches in plastic can be real nasty. For some plastics that don't sand well consider chemical strippers, be gentle, or go up to 110 or 180. 8" rotary is my choice for stripping.

Prep for filler: 80
No reason to go finer or coarser for any kind of filler that I can see. Often I'll use a grinder to prep for filler, sometimes a rotary sander, sometimes hand.

Prep for primer: 220 dry
Surfacer should fill 220 grit scratches easily, since you don't want to put any waves into the work at this point you'll probably want to use a block. The right way to block sand is in an X pattern, overlapping, never spend time on one spot, be aware of where you're making contact with the peice and for what duration. You can start coarser, 40 if you need to do some serious shaping, but as you progress you should use finer grits and finish with at least 180. This is the stage you're shaping filler in, so I should mention that the best way to do this is clearly with a body file, a cheese grater tool that's used shortly after the filler kicks but before it fully cures. It removes a massive amount of material easy as pie so you can finish with sandpaper. This is also the magic time window to trim fiberglass, do butt joints in cloth, stuff like that.

Prep for topcoat: 400 wet or dry
Minimum, some say you can see 400 scratches, don't go more than 600 or you get adhesion problems. Again, you'll want to use a block, or a sponge on curved surfaces so you don't put waves in the peice with your finger. The purpose here is to either take small imperfections in primer (in which case you can start with lower grits like 220 and work up to 400), or to just provide tooth (just scratch open the surface and you're done). You might want to use a guide coat, a spray can of a contrasting color so you can see all the low spots and keep removing primer till you remove the color (and hence the imperfection).

Prep for buffing: 1500-2000 wet
Don't start with less than 1500 unless you have lots of material to work with or some serious imperfections. Finishing with either 1500 or 2000 is fine cause the buffer can pick it up from there. I guess some guys go way finer than this but I don't see the point. Every once in while you can stop and remove the water with a squeegee, or better an air hose, and pull some light over it to clearly see your work. High spots will be sanded dull, while low spots will remain glossy and reflective. You use light here just like a guide coat. Keep going till it's evenly dull, use a block or sponge never your bare hand. Some use DAs but I don't. Watch out for burn throughs, if you sand through clear coat or through to primer then you've lost a lot of time to fix it.

That's the basics of choosing sandpaper for different tasks as I know it. If it gets stickied maybe we can feel justified in ridiculing people that ask questions about which grit to use. Please post your thoughts, I'm sure others do it differently or would have something to add.



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PostPosted: Thu Nov 25, 2004 11:01 pm
Aberrant did a nice job of explaining this topic. ?I will add that in recent years , other grading systems have appeared in North America (That includes Canada Aberrant..) the most prominant one being the p grading system...Soooo...That means that you will probably see a lot of the CAMI grading...(The one everybody knows about) and ther FEPA, or P grading system (The new kid) The glitch is that the numbering systen is very similar, but the grades are just enough different so they can throw you off ....especialy above 220 grit.



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PostPosted: Sat Nov 27, 2004 12:11 am
I don't know what grade I used but I sanded to 220 in a lot of places and that is as fine as I got and it looks fine, no sand scratches that I can see. I think this is something that wouldn't work with a BC/CC though, I used a single stage omni paint.

Excellent info though! I still haven't buffed my car, so anything in that department really helps me. I got some runs to get rid of, not sure if I want to kill myself with the 1500 grit removing them or just go 220 to 400 to 600 to 1500 or ?

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PostPosted: Sat Nov 27, 2004 1:14 am
220 for polishing is crazy low, you'll never get those scratches out. It's best to scrape them down with a blade before it cures, but too late now. I'd start at 1000.

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PostPosted: Sat Nov 27, 2004 9:25 am
ok the next guy that reads this and DOESN'T VOTE=
we are gunna drive over to your house and paint your car bright pink with hot lime green polka dots, and mustard yellow lettering that says "I go to an autobody forum, and ask many, many questions, but when they ask ME for something, I just ignore it"

100 views, and only 8 votes!?!?


YOU GOTTA BE KIDDIN ME!


do your part

nuff sed
"I believe the state of arkansas flag colors should be,...................PRIMER"



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PostPosted: Sun Nov 28, 2004 11:26 am
Great post, Aberrant!
Thanks for taking the time to explain this to us noobs. It answered some questions for me. I just ordered a crap load of 3m 320 grit, thinking this was the correct grit used for final sanding of the surface primer. :( Wished I'd seen this post before hand.

Yeah, make it a sticky. Or better yet, put it in the instructional section.

Mike



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PostPosted: Sun Nov 28, 2004 11:40 am
I pretty much agree with most of the reccomendations.

I use 50 to prep and shape filler

500 wetsand to prep for topcoat (i.e. blending)

1000 followed by 2000 prior to polishing.

And yes, I voted! :wink:



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PostPosted: Mon Nov 29, 2004 8:53 pm
Great info...Should be sticky



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PostPosted: Wed Dec 01, 2004 12:59 pm
I voted, I voted....please, no pink paint...



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PostPosted: Fri Dec 03, 2004 4:18 pm
GREAT post! this should be stickied for all us rookies out there! I did vote so please answer this question :lol: :
If I put on a coat of clear and then don't come back to it for a day or so, what grit should be used to put some "tooth" in the clear to apply more coats of clear or am i off my rocker and causing myself more work.
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