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Unread 01-23-2012, 09:36 AM   #1
Ken4444
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Correct sandpaper grit for bare and painted metal before priming?

In general, what is the correct final grit of sandpaper to use before priming? I'm facing three different scenarios with my tub interior repaint:

1) On bare metal, what grit should it be sanded with? The metal has been stripped with paint stripper and strip disks, so it has visible scratches all over it. I'm thinking I need to sand it with something finer to lessen the scratches.

2) I've repaired several spots with new sheet metal and body filler, and those are sanded smooth up to 400. Should I go back and rough those up with something more coarse before priming?

2) My rollbar is in good shape and doesn't need to be stripped to the metal. So I need to sand it just a mininmal amount. What grits of sandpaper should I use before priming?

My plan is to spray PPG epoxy primer and then PPG Deltron DBC 2000 (single stage) color match paint. I am only painting the interior of the tub at this time.

Thanks!

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Unread 01-23-2012, 07:52 PM   #2
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you shouldnt have to worry about the scratches in the sheetmetal before priming. the primer will fill in the majority of the scratches. sand the primer with 320. same for the roll bar. if you want to do a second coat of paint, i recommend wet sanding with 1000-1500.
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Unread 01-23-2012, 09:27 PM   #3
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I just got done going through repair/refurb thread (scrolled through, looked at pics, read a little- got the gist) nice work. 320 should be good for roll bar. Be aware that epoxy wont fill large grind marks and or course sanding and they will show in the epoxy and your paint. I understand after replacing all that rust in the tub you want the corrosion protection epoxy will provide. My advice would be to epoxy the whole tub, Then on top of that prime w/ a 2K urethane, sand then paint.
You will find that epoxy is tough to sand and it will gum your paper, unless wet sanded with finer grits. By using a 2k urethane you will fill small scratches and be able to quickly sand out anything that comes through, and it will be it will be easier to sand the primer than bare metal.

I would also recommend looking into some of then lower lines of PPG paint. Deltron is great stuff, but pricey. If your supplier is a platinum retailer ask about the PPG shopline products, good stuff , good value. The omni line is pretty similar and pretty good as well.
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Unread 01-24-2012, 07:18 AM   #4
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Thanks for the details.

Quote:
Originally Posted by drroy View Post
... By using a 2k urethane you will fill small scratches and be able to quickly sand out anything that comes through, and it will be it will be easier to sand the primer than bare metal..
Okay, good information. So you're reccomending epoxy primer, then 2k, then sand, then paint.

I've seen this method (epoxy, 2k, paint) mentioned several times before around here.

Quote:
Originally Posted by drroy View Post
I would also recommend looking into some of then lower lines of PPG paint. Deltron is great stuff, but pricey. If your supplier is a platinum retailer ask about the PPG shopline products, good stuff , good value. The omni line is pretty similar and pretty good as well.
My original plan for epoxy primer PPG and Deltron was based on the reccomendation of the shop where I'll be buying the paint. So I need to go back to them and have them reccomend a 2k primer that's compatible with whatever paint I'll end up with.

I had finally accepted the fact that I'd be paying $100 a quart for primer and paint because I don't mind buying great quality paint if it will produce a better result. However I will see if they can quote the job with the Shopline products and see what that does to the price. If I'm having to buy 2 quarts of primer then switching to a more value-oriented product line might be acceptable.

Thanks again for the info!
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Unread 01-24-2012, 02:24 PM   #5
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Definitely look into shopline if they have it, if not ask about Omni, there is good savings to be had over deltron and you are not sacrificing quality, it is all good stuff. And to be honest If this is going to be your first time auto body painting and using a HVLP gun, the marginal increase in product quality will be lost on you. Its not a dig on you, you are clearly skilled, but there is a learning curve to this stuff, and everybody starts out doing less than show quality work, and using deltron will only add the cost with little to show for it.

With that said, whatever product line you decide to use go to the PPG web site and download and print all the PDF technical data sheets (TDS) for those products and stick them in a binder to keep in your shop as a reference. It will be helpful to know whats available, mixing ratios, product compatability, curing times. Ect.

https://buyat.ppg.com/refinishProductCatalog/

just curios what gun(s) are you using and how is your compressor?
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Unread 01-24-2012, 07:34 PM   #6
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1) On bare metal, what grit should it be sanded with? The metal has been stripped with paint stripper and strip disks, so it has visible scratches all over it. I'm thinking I need to sand it with something finer to lessen the scratches.
-Bare metal I would sand up to 400 then shoot it with a primer. I would shoot a couple coats of primer then sand the primer smooth and try not to sand through all the primer, then paint.
-If the visible scratches are not blending well with the 400 grit then try 240 then 320 finishing with 400.

2) I've repaired several spots with new sheet metal and body filler, and those are sanded smooth up to 400. Should I go back and rough those up with something more coarse before priming?
-The 400 grit paper will be fine with the body filler. A good idea before priming would be to go over the surface with a red scuff pad made by 3m. It will help adhere the primer to the body filler and any other areas not sanded thorough enough.

2) My rollbar is in good shape and doesn't need to be stripped to the metal. So I need to sand it just a mininmal amount. What grits of sandpaper should I use before priming?
-If you want to just repaint I would go with a 800 grit or 1000 grit sand paper. But if you have chips that you are hopping to cover then i would sand them smooth to a blend with the rest of the paint with some 320 grit. Then prime those areas, followed by some 400 grit sanding to smooth the primer, finally paint away.


If you have any other questions feel free to PM me.
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Unread 01-25-2012, 07:19 AM   #7
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Quote:
Originally Posted by Tjcrawler View Post
...If you have any other questions feel free to PM me.
Thanks for the advice. From your post and others, I have a good idea of how to proceed. Thanks to everyone to replied. I will be posting pics in my build thread. I hope to start spraying primer in a couple of weeks.
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Unread 01-25-2012, 07:33 AM   #8
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Quote:
Originally Posted by drroy View Post
...And to be honest If this is going to be your first time auto body painting and using a HVLP gun, the marginal increase in product quality will be lost on you.
I agree. I can see the logic in that. It'd be like putting a first-time driver in a $90K sports car with a 6 speed manual transmission.

Quote:
Originally Posted by drroy View Post
...

https://buyat.ppg.com/refinishProductCatalog/

just curios what gun(s) are you using and how is your compressor?
Thanks for the link! Man... I never knew PPG would encompass so many different brands! 11 brands for "refinishing" alone. That's one of those things about the auto body world that adds to the hocus pocus, black-box element in my mind. I can see why a painter or shop might tend to stick to one brand or another as long as they've had success with it.

I'll be using a HF HVLP gun with a neighbor's 7 gallon, 3 HP compressor.
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Unread 01-25-2012, 10:28 PM   #9
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I would consider upgrading your compressor size, or rethink your gun selection. I have heard the HF guns are not that bad, but regardless HVLP guns are air hogs and you will be spending a lot of time waiting for your compressor to catch up and build pressure with a tank that size. Plus if the compressor is running 100% duty cycle you will end up with a lot of moisture and condensation in your lines and paint. Look into a conventional gun or LVLP gun. Or at least a second compressor to tie into.

An abundant, cool, clean and dry compressed air source is realy important in getting good results. As important as gun and paint selection.
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Unread 01-26-2012, 06:15 AM   #10
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Quote:
Originally Posted by drroy View Post
..An abundant, cool, clean and dry compressed air source is realy important in getting good results. As important as gun and paint selection.
Yeah, that was my reason for painting during the winter when we're more likely to get weather in Houston with dry air. Normally our climate is very tropical and humid, but in the winter we have more dry days.

I am working on lining up a second compressor for the job..
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Unread 01-26-2012, 04:41 PM   #11
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Originally Posted by Tjcrawler View Post
1) On bare metal, what grit should it be sanded with? The metal has been stripped with paint stripper and strip disks, so it has visible scratches all over it. I'm thinking I need to sand it with something finer to lessen the scratches.
-Bare metal I would sand up to 400 then shoot it with a primer. I would shoot a couple coats of primer then sand the primer smooth and try not to sand through all the primer, then paint.
-If the visible scratches are not blending well with the 400 grit then try 240 then 320 finishing with 400.

2) I've repaired several spots with new sheet metal and body filler, and those are sanded smooth up to 400. Should I go back and rough those up with something more coarse before priming?
-The 400 grit paper will be fine with the body filler. A good idea before priming would be to go over the surface with a red scuff pad made by 3m. It will help adhere the primer to the body filler and any other areas not sanded thorough enough.

2) My rollbar is in good shape and doesn't need to be stripped to the metal. So I need to sand it just a mininmal amount. What grits of sandpaper should I use before priming?
-If you want to just repaint I would go with a 800 grit or 1000 grit sand paper. But if you have chips that you are hopping to cover then i would sand them smooth to a blend with the rest of the paint with some 320 grit. Then prime those areas, followed by some 400 grit sanding to smooth the primer, finally paint away.


If you have any other questions feel free to PM me.
bare metal- shouldnt be sanded with anything finer than 180 grit, i actually prefer 120 grit. 400 grit is too fine, even if an epoxy primer is used......ask me how i know......i did a nose on a grumman bread truck, sanded the bare aluminum with 400 grit, epoxied(as a sealer) and sprayed single stage on it. guess what was back in the shop 2 weeks later with almost all the paint blown off from stone chips....the grumman van.
second off; when you do a a spot of body filler, your rough out work is up to 36/40 grit paper, second coat of b/w is 80 grit, and final putty/glaze is done with up to 180. you use 400/320 for your feather edge to spray the primer into. usually all body shops have only a few grits of paper they use. it cut alot of cost down.....rule of thumb on grit paper to use when ya start a project........36/40, 80, 180, 320/400, 800wet, and 2000wet. they are really all you need to do any project.
your roll bar; with nicks and scratches sand with 180, without nicks and scratches 320/400. you will do more damage trying to feather small spots with 400 then with 180, plus it'll take ya twice as long.
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Unread 01-26-2012, 06:23 PM   #12
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Quote:
Originally Posted by JHW12084 View Post
bare metal- shouldnt be sanded with anything finer than 180 grit, i actually prefer 120 grit. 400 grit is too fine, even if an epoxy primer is used......ask me how i know......i did a nose on a grumman bread truck, sanded the bare aluminum with 400 grit, epoxied(as a sealer) and sprayed single stage on it. guess what was back in the shop 2 weeks later with almost all the paint blown off from stone chips....the grumman van.
second off; when you do a a spot of body filler, your rough out work is up to 36/40 grit paper, second coat of b/w is 80 grit, and final putty/glaze is done with up to 180. you use 400/320 for your feather edge to spray the primer into. usually all body shops have only a few grits of paper they use. it cut alot of cost down.....rule of thumb on grit paper to use when ya start a project........36/40, 80, 180, 320/400, 800wet, and 2000wet. they are really all you need to do any project.
your roll bar; with nicks and scratches sand with 180, without nicks and scratches 320/400. you will do more damage trying to feather small spots with 400 then with 180, plus it'll take ya twice as long.
I do work in a body shop. HAHAHA, in los gatos. When using a real rough grit you leave tear marks. Using the finer grit will smooth out the scratches and be invisible.
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Unread 01-26-2012, 08:23 PM   #13
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http://www.google.com/url?sa=t&sourc...UpelcEvs2FrdoA

Some info. For later if you decide to paint the exterior I have gotten good results with 2 small compressors tied in together and a fiter, painting small jobs like a fender / door. started having problems with larger jobs (whole car)
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Unread 01-28-2012, 05:48 PM   #14
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I do work in a body shop. HAHAHA, in los gatos. When using a real rough grit you leave tear marks. Using the finer grit will smooth out the scratches and be invisible.
i do body work too......for 14+ years......theres no 'tear' marks when you sand with 180 and prime....then sand with 400.....i'd pay to see someone strip a hood down with 400.....cause it'd be a long day, very long.
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Unread 01-31-2012, 09:18 AM   #15
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i do body work too......for 14+ years......theres no 'tear' marks when you sand with 180 and prime....then sand with 400.....i'd pay to see someone strip a hood down with 400.....cause it'd be a long day, very long.
I agree...Except I take it a bit further with 80 or 100 grit if re-priming....Any good epoxy/2k or hi build primer will bury an 80 grit scratch no problem....And your getting MUCH better adhesion than sanding with a finer grits.....Primer should be the strongest layer in the whole system, why skimp on adhesion right..lol... I also sand primer with 400.

I see folks mention sanding with 1000 and higher...Not me, anything over 800 is for polishing only, fine grits offer very little "tooth" for new paint to hold onto. And if you are out of your flash "window", you arent getting much of any chemical adhesion...Sanding is pretty much the only thing holding the layers together.

I'm an Airbrush artist and custom painter.. Feel free to check out my websight to see if my methods yield any "tear" marks and such....(not trying to spam my goods, just showing that I'm not making things up), haha

http://www.artbydell.com/
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