Final sanding grit before primer?

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PostPosted: Thu Apr 21, 2011 9:26 pm
nothing wrong with filling 80 with primer.....plenty of guys do it with no problems. its all in the application and drying process.

if you are getting shrinkage under your paint by doing this, then you are doing something wrong. the stuff shrinks over any grit scratch, you have to let it before you continue on. flash times flash times flash times.

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PostPosted: Thu Apr 21, 2011 9:52 pm
Tim1137 wrote:Most of my panels are already 220'd. If i just use a red scotch pad after the 220 will I be ok? Im just worried my 220 scratches will show through in the end.

Red pads are the equivalent of 320-400 grit sandpaper, but I'd use the 320 paper myself
unless the panels are irregular (such as a firewall or doorjambs).

More than one way to skin a cat though.
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PostPosted: Fri Apr 22, 2011 1:38 am
Remember, Im not leaving any 80 grit scratches in the surrounding painted area, just the bondo. (Thats where you get a problem from, not from the bondo)


Interesting ODG. Why does the primer fill 80 grit scratches in bondo, but not the surrounding areas? Also when you say you feather the surrounding areas with 360-400, does that mean any surface area without bondo is sanded down that smooth before the 2k?

BTW, is that a tarpon in your logo?

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PostPosted: Fri Apr 22, 2011 6:00 am
Im not sure why it does it, but every one I have ever fixed that had sand scratch was in the surrounding area where they didn't feather the edges around the bondo, not in the bondo.

After I feather mine with 320-400 Ill blow it off real good and then wipe it down with W&G remover before I apply the primer.
Ill spray the first coat and let it flash real good. They teach you this in school not to rush the first coat. Then Ill spray the second and third coat with plenty of flash time. I always let it dry over night.
Seriously,,,I have never had a problem with sand scratch..........................
Like I said,,,you have to get rid of the scratches from your 36 or 80 grit that's around your bondo. And I final sand the bondo with 80.

You guys that worry about seeing sand scratch from 220,,,worry to much.......................


(Thats a trout in my banner Edpol)
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PostPosted: Fri Apr 22, 2011 12:36 pm
Thanks. After stripping, I suspect I'll find some bondo under the old paint on this 49 year old car. Found a ton of it in the engine compartment alone, where patch panels were crudely welded in. In this case, I had to remove the bondo, because I'm replacing the patch panels with ones that actually fit. When I get to the exterior, does the old bondo have to be removed, or can I just go over it if it's still adhering tightly?
If I understand correctly, 2k needs time to shrink, so I planned on priming all my parts and the body with one coat of 2k, then waiting until the next day for the second coat, another 4 -6 hours until the 3rd coat. I'll let the primer sit for at least a week while I work on the engine and tranny. I can wait between coats, because I'm also painting a few rooms in the house.
The plan was to strip the paint, do any necessary body work, epoxy prime. Add filler where it needs it, sand with 80, then 180, then shoot the 2k. Should I add sanding with 220 and 320 before the 2k?
Maybe my memory is bad, but prepping seemed a little simpler back in the day.
(Trout? On my crude monitor, the scales look very large. I would never have guessed it's a trout.)

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PostPosted: Fri Apr 22, 2011 3:54 pm
You asked ,"When I get to the exterior, does the old bondo have to be removed?"

Most of the guys and I strip off all the old bondo and rework the area with fresh bondo. Sometimes you will find hidden rust.

You said something about spraying one coat of 2K primer and waiting until the next day for the second coat. That's not needed. The 2K primer is not designed to be used that way. You spray on the first coat, let it flash for about 30min, then spray your second coat about 20 min then your third.
If you wait until the next day to spray your second coat, you will need to block it. You want millage when you spray your 2K primer. Its designed to fill in sand scratch and to perfect your body work with a thick layer.

"Some" of you guys go WAY overboard with details that are working you to death and/or waisting materials......

O-Yeah,,,here is a pic of a small tarpon. There scales are a LOT bigger.

Image
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PostPosted: Fri Apr 22, 2011 4:12 pm
Thanks. You mentioned earlier about giving plenty of time for the first coat to flash, so I thought I'd have enough time to do other things while waiting. I'll put 3 coats on the same day, then go to the other stuff. That should allow plenty of time for shrinkage before block sanding.
Good point about rust that may be under the old bondo, especially if it was applied by whoever did the engine compartment. I'm finally done with that, after altering the patch panels so they match the original metal. Took a long time, because I couldn't use the mig with gas, and had to go with flux core and sticks. 7014's @ 20 amps actually worked better than the flux core.
But I digress. So what should I use on the epoxy? 80, then 180? Or is it better to go finer before the 2k?

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PostPosted: Fri Apr 22, 2011 5:19 pm
Check with the "Epoxy Experts" on this site,,,but I would use something like 320grit
"The number of parasites in the USA has now eclipsed the number of productive members of society"


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PostPosted: Fri Apr 22, 2011 6:52 pm
Thanks. Sounds good. I forgot that you use etch primer.
On my monitor, the scales on the trout look as big as the ones on that tarpon. Time for a new monitor.
That small tarpon looks like our local shad on steroids. Now I have a craving to go fishing. The stripers are in, and tonight looks like a good night for 'em.
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