What grit to use?

General Discussion. Make yourself at home...read, ask and answer!
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I don't care, I already know how to sand.
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PostPosted: Mon Mar 23, 2015 7:59 am
wahoo wrote:So I put my filler on bare metal before epoxy, how high of a grit do I have to sand to before I epoxy over it?



Cut it flat by block sanding with P80. Then go over it again with P180 on a block to feather the filler out and remove the P80 scratches.
Now you can spray your epoxy. 2 coats for bare metal, 1 or 2 coats for filler.



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PostPosted: Tue May 03, 2016 2:19 pm
Can you confirm this is the correct grit progression?

EPOXY
P80 - strip paint / bare metal prep for epoxy
"red" scuff pad - prep for filler/primer in contoured areas
P180 - existing paint prep for epoxy / prep for filler/primer (if epoxy has sat for 7+days) /

2K URETHANE PRIMER
P180 - initial primer block sand
P320 - block sand primer
P400 / Red Scuff pad - prep for sealer

SEALER
P600 - prep for basecoat

BASE COAT
NA

CLEARCOAT
P1500 - wet sand clearcoat
P2000 - wet sand clearcoat

Also some some specific questions:
1) Should I add an interim step to my primer block sanding (e.g. P220)?
2) At what grit should I move to a wet/dry paper (P400 or 600)?
3) Which will I use the most of? In other words, which should I purchase in bulk rolls vs sheets?
4) Besides 80 grit, what else should I get for my DA or Random Orbit (RO) sander? Which steps would you use a RO on?
5) My RO has a 5-inch pad, should I upgrade to a 6-inch? PSA or hook & loop?
6) Favorite blocking boards/tools?



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PostPosted: Tue Nov 29, 2016 8:27 am
I usually shoot my sealer and base coat wet on wet and do not sand the sealer.



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PostPosted: Tue Dec 06, 2016 7:52 am
gpracer15.... What are you using for sealer?



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PostPosted: Sat Apr 01, 2023 2:56 pm
Great Site Ihav learned a lot already!
I bought a wrecked 2008 Corvette which needs fenders and a front bumper to start.
Both need to be repainted! The fender is carbon fiber and bumper is urethane(?)

1, Do I need to remove the paint or will just sanding away the clear work?

2. Recommendations for sand paper ( grit) ?

White to silver on the fenders,

Black to silver on the bumper

Thanks for your help!

Tom



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PostPosted: Wed May 24, 2023 4:48 am
I'm new and audaciously would like to add to this content which I think is great. So I want to transfer some of my woodworking knowledge in terms of SANDING which probably apply the same in metal/plastic autobody.

I have just read this whole thread and I see a lot of questions about "what grit to go to next". First know that Sanding is a SYSTEM. Technically you should move up to the next available grit. There are many reasons for this. This is because if you start with 80 and go straight to 220.. It will just cause problems some of which you may not realize.. (imagine if you went from 80-400 for example as it would be pointless and eternal)
SO a quantum leap 3-4 paper grades up results in..:
1) It will take forever and all day to cut back down thru the 80 grooving to new much finer base grit.
2) You will likely miss some a lot of gouges like this. Its important to understand this because you may move from 80 to 120 and it takes minutes to achieve uniformity. So you create a false TIME expectation in your mind. And may feel like you have "Sanded long enough" in future circumstances where you attempt big "grit-leaps".... Only to find you were not even close.
3) Making big grit-jumps will actually take you longer, you will have to work harder, and it will cost more/ because the finer the grade, the longer it takes, and finer paper dies much quicker. So we are dealing with Orders of Magnitude in terms of wa$te when "grit jumping"..
4) The moral, regardless of WOOD or METAL I think is never go lower than you absolutely must. In fact I kinda see it as a real question of whether its worth considering chemical strippers for many imaginable auto paint repair scenarios. I honestly can't stand to use them though. Maybe I am just too much of a stickler, but I imagine a scenario where someone is painting a fender from a salvage yard to replace in an auto clear coat system and it's chipped, flaked and rusted to the metal in some areas. Personally I think that qualifies as an on-bench chemical strip preferred. Perhaps I am being too strict and the reality is that people just work the high damage areas and Grit-grade the rest with good transitioning while praying that the rest remains stuck. I'm would guess more often than not its the second scenario as the general integrity levels in folks seem to DEGRADE before our eyes (Digressing ohh NO).
5) With all that said, remember that just because you are using a really low/rough grit for a repair area DOES NOT MEAN to take that rough grit even an inch further than necessary.! Consider work area all independent on a micro level of the whole macro so to speak, Use nothing more aggressive that the area requires.

It is my consideration of how to approach some new auto body parts I just purchased which I have found to have come with a really good E-Coat on them which is prompting a lot of thought and again I am getting great help at this site.

Obviously there is a lot I don't know about auto body fillers and high-build primers, and general body repairs. But still consider with regard to the high-build primers, you are still laying mountains on top of mountains that you have to trim off, so to speak. And the higher the mountains the more coats, scraping of the tops, and WORK to bring the new peaks back down to uniform level somewhere in the middle. I do see the post above advising a 180 grit prior to high-build primer application. which I consider still pretty gnarly. And 180 is still a long way from grinding out rust or shaping with a 36. Just making a point.

OR am I completely off base and is it simply required to have no higher than 180 grit for the product to stick?? And we are just therefore forced to do all this work if a job requires a high-build?? So teach me if ya would be so kind. I AM READING AS FAST AS I CAN AROUND HERE AS WELL..

In terms of autobody metal, I also do not yet understand how certain products adhere. I would imagine there are filler types ranging from what I know as bondo and so forth onward. And that many of them probably require deep "gnarly" cuts to keep the product from simply POPPING OFF. As well as whether a filler require priming or specifies, not to, or use a certain type with this, etc...?! ... So I get all that too. Probably not as good as I would want to believe.

Contrarily, In terms of finishing wood floors per say, the opposite even applies somewhat as you can have some pretty nasty grit-work and get away with a thick urethane finish product and GRAVITY will simply bring it all to level no bubbles and plush (stain and wood type not considered). Of course all these factors including "high filling" a urethane floor finish still affects the final "Look", but does it matter all that much on a surface made for dog paws and todays man-hating power heels? LOL... And in fact, Contrary to automotive application, you can get wood floors TOO SLICK if not careful and turn your living room into and ice rink. So we are still apples to oranges I can see/ for the most part.

I'm just a new guy that knows something about surface prep (including cleanliness) mostly in terms of wood, with a little metal experience or varying constructs, but I start to realize that there are more similarities than I see up front. So plenty of RESPECT out to you all. And thanks to those taking the time to read my ADD induced posts.

Something else that I see in this thread that may be overlooked so I must bring it up. There may already be a thread about safety and if there is not there should be feel free to transpose this to kick one off. Because all this talk about sanding is important and I will tell you why in a minute.

MODS - IF this safety tangent is considered too off topically hi-jacking, do what you want with it. Trash here down, leave it, or kick off a safety thread. I just have to say my piece here else I would feel negligent. Its like seeing a dad in Lowe's on the other end carrying a TREATED 2x6 just dripping with GREEN WET COPPER. Dads thinks he really making kiddo part of the team. The truth is they need to run home and get in the shower cause a little copper goes a long way.!! You get the point.

A QUICK WORD ABOUT SAFETY... Which I am pretty good at in terms of TELLING. Doing is another thing LOL.. One of the most important moments in my life was one day picking up some really strong paint stripper from the actual paint manufacturing company (we are talking THE HORSES MOUTH) and I was like,
"so as long as I can't smell it I am safe right?".
The guy blew me away with honestly and said,
"NO in fact the worst stuff you usually cant smell, or does not smell like what you would think the bad parts should smell like, and is lying in there beneath the noxious fumes that are readily identifiable to your nose. And this particular stripper is loaded with the worst"..
I was like WOAH!! Then I says
"so a good top notch respirator will do it then right?"
He was like,
"NOPE and fumes go right thru the best filtered respirators, and the only way to be protected is a space suit basically with an air supply from somewhere else."

Anyway there goes my favorite warning "if you can smell it you are ingesting it". Cause apparently there's a bunch you can't smell the we can ingest as well... A POINT I am trying to make in safety is as follows. The first thing I am going to do, and by professional background, is look up product SAFETY & DATA SHEETs. And some if these products on this particular auto project ranges from primers, to cutter, to color coats, to clears, hardeners, accelerators -ALL OF IT FREAKS ME RIGHT OUT..

My ADD is bouncing again. What surprised me was that I saw data indicating that certain products were actually MORE Dangerous once dried and while being sanded. Logically you would think that once all the "solvent" evaporate out to God's Green Earth that you are safe right? Other than a little inert powder remains you could just cough up in the shower. That was not so correct as I started seeing data in terms of Death based on Gaseous state vs. Breathing the solid sanding debris. And while you can not escape fumes, Sanding debris can def be avoided for the most part with a good respirator. Reading the material it looks almost like some of the really bad stuff turns to solids when applied, or always was molecularly large enough to be considered a solid and just saturated into the solution as sold/ and for a minute. I think I read one of these chemicals even uptakes systemically to the body from the sanding dust landing on skin. Now I could be wrong but it got my attention for sure. I'm not challenging anyone and I am not referencing it either. The data is available with every product and I don't see any reason to technically catalogue the entire industry ingredients lists right here at this site. And while this may seem silly or redundant to you pro's here, perhaps its not. But I could def foresee the DIY guy coming around here, going out back with his new gun and paint, and failing to take the correct precautions. LIKE ME in any other life..

So again I am not hijacking THIS THREAD, NOR my own POST. This is a thread about sanding so I feel like it's merited. I have not yet found any safety based threads here so point me if there is one, and always feel free to correct me. My web forum policy if I am wrong is thank you for increasing and clarifying my knowledge base and if I made you look to learn something then we all win a prize.

And its not my intent to rub too hard on a soft subject, but sometimes I walk in that autobody paint store and the fumes are so thick in the air I am just thinking SWEET JESUS and their noses are probably numb to it. And while the human body can tolerate a whole lot, I would not even want to look at statistics for body guys from back in the day. Honestly, these paint mixing places should me working their products under medical grade air hood ventilation systems. Its like OSHA just forgot about an entire sect of society. That's as political as I will dare on that.. And NOTHING Pisses me off more than walking into Lowes that has all the lawn chems in the store. GOOD GRIEF.!!

Sorry couldn't sleep..
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