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Old 12-28-2009, 11:01 AM
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Sanding compound curves...

I'm trying to smooth out the primer on the back corners of my truck and am having some trouble.

I feel like using a sanding block is going to create flat spots that will show up after I topcoat (black, no less.) I tried to use a flexible block, but as the curve changed, it caused wrinkles in the sandpaper that cut grooves in the primer.

Are there some techniques/tools I'm missing here?

Thanks
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Old 12-28-2009, 12:36 PM
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You need a set of Durablock sanding blocks.

Vince
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Old 12-28-2009, 12:49 PM
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I'm using Durablocks.

I'm struggling with the technique of using a flat block to make a curve. When I sand, there seems to be no way to do anything but a thin strip at a time because of the acute angle of the cab. I'm concerned that those strips are going to show when I gloss it.

Maybe I just need a little more practice...
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Old 12-28-2009, 01:04 PM
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Durablock Scuff Block

Try this one. Its quite flexible. I use it on sharper curves.

AF4405 Scruff Block: This block is designed for sanding and prep work. It allows for easy use and much improved durability and firmness. Size: 1/2"H x 2-1/2"W x 5-3/8"L
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Old 12-28-2009, 02:41 PM
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Over the years I have tried many different methods for sanding panels such as yours and I still revert back to what I did years ago while working on vintage cars everyday.

This is an odd concept here, stay with me. I look at the curved panel as what I call a "Curved flat panel". I treat it the same as I would a flat panel, only I sand just a very small portion of it at a time. I feel for a high spot and sand it down, then find another high spot (usually VERY close to where you were just working) and sand it down. A slight "roll" on the block, but basically that's it. I use a regular 5" rubber block usually.

The trick being LOTS of feeling and VERY LITTLE sanding. Sand the high spot very carefully and lightly, then feel it. Over and over, until the whole area is done. I know it sounds odd, but I am darn sure I could sand a block of filler into a bowling ball if I wanted to using this method.

A while ago I did a set of 72 VW Bug fenders using this method and they turned out killer.

Brian
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Old 12-28-2009, 04:28 PM
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Don't you have a Durablock in your selection that is about 1" wide and 3/4" thick, and very flexible? It will bend around a compound curver easily.

Vince
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Old 12-28-2009, 07:30 PM
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MartinSr: That's an interesting concept. I'm going to give it a try tomorrow. I also have a DA with a soft interface pad, but I wonder if that would just be opening a can of worms. I may try it when I move to 320. I'm at 220 now.

Z28: I do have that block and it's the first thing I tried. Because the severity of the curve varies so much, I had to keep bending the block back and forth, which caused wrinkles in my paper, which in turn caused a few disastrous grooves.

Part of this may be the fact that the paper doesn't stick very well. Can't get my garage over 49F. Wyoming is a winter wonderland you know...
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Old 12-28-2009, 08:34 PM
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Can you try using a pint stirrer stick. Wrap your paper around it several times. It helps if the stick is wet so it bends easier.

Vince
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Old 12-28-2009, 08:58 PM
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Quote:
Originally Posted by wyomingclimber
MartinSr: That's an interesting concept. I'm going to give it a try tomorrow. I also have a DA with a soft interface pad, but I wonder if that would just be opening a can of worms. I may try it when I move to 320. I'm at 220 now.

Z28: I do have that block and it's the first thing I tried. Because the severity of the curve varies so much, I had to keep bending the block back and forth, which caused wrinkles in my paper, which in turn caused a few disastrous grooves.

Part of this may be the fact that the paper doesn't stick very well. Can't get my garage over 49F. Wyoming is a winter wonderland you know...
That one looks like fun, I have to wonder if your primer is fully cured being that it is so cold, it might make it too soft for blocking hence the gouging.
I also had problems with Omni primer not setting up, but if it's not clogging your paper then it's your technique.

I'm not a fan of durablocks, I find them cumbersome to use, and I never found it to be a asset on anything outside of scuffing the inside of a truck box floor.

I would use 8" straight block around the corners, then I would use a soft 5" around the apex edges to smooth out the roundness, patience is key.
If you feel confident enough to use a DA, make sure it's a finish DA and not a grinder/DA sander.
A palm sander, or a Hutchinson works well, 5" disks might make it easier if you have one, but 6" works too.

220 grit is perfect for sanding primer that is going to be re-primered, but if you are going to go to a sealer or a base coat next step, stick with 320 or 400.

Or you could go 600 grit wet sand with a softer backing pad to finish it off, if you soak the paper in warm water and add a touch of dish soap it should help avoid gouging the primer.
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Old 12-28-2009, 09:11 PM
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If you have an ACE hardware nearby you can ask if they carry:

"GatorGrit Premium Plus+ ultra fine finish"(320 grit) sanding sponges #3570

the grit is bonded to the sponge, no paper, no wrinkles. Exactly perfect for exactly what you are doing. Guild coat and patience.

Have fun with it.
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Old 12-28-2009, 09:34 PM
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Quote:
Originally Posted by oldBodyman
If you have an ACE hardware nearby you can ask if they carry:

"GatorGrit Premium Plus+ ultra fine finish"(320 grit) sanding sponges #3570

the grit is bonded to the sponge, no paper, no wrinkles. Exactly perfect for exactly what you are doing. Guild coat and patience.

Have fun with it.
I forgot about those, good call.
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Old 12-28-2009, 09:56 PM
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Quote:
Originally Posted by cutterbond
I have to wonder if your primer is fully cured being that it is so cold, it might make it too soft for blocking hence the gouging.
I also had problems with Omni primer not setting up, but if it's not clogging your paper then it's your technique.
I actually have had a problem with my paper clogging as though the primer wasn't fully set up in places. Impossible, though. I painted the thing in the middle of the summer and it sat around for months in 80 degree weather.

Overall, I wouldn't use the Nason 2k again. It was really hinky to use and now I'm just praying there's nothing wrong with it that could cause me problems down the road.
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Old 12-28-2009, 10:04 PM
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Quote:
Originally Posted by wyomingclimber
I actually have had a problem with my paper clogging as though the primer wasn't fully set up in places. Impossible, though. I painted the thing in the middle of the summer and it sat around for months in 80 degree weather.

Overall, I wouldn't use the Nason 2k again. It was really hinky to use and now I'm just praying there's nothing wrong with it that could cause me problems down the road.
Is that Nason the yellow primer?
Because I have a friend that uses that, and I was pretty impressed by how fast you could sand it and not have it ball up the paper, and it had a nice fill to it to.

I was thinking about switching to it, because K36 takes so long to cure.
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Old 12-29-2009, 12:25 AM
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Quote:
Originally Posted by 302 Z28
Don't you have a Durablock in your selection that is about 1" wide and 3/4" thick, and very flexible? It will bend around a compound curver easily.

Vince
I have blocks coming out of my ears, from Durablocks to home mades. The thing is if you are running this flexible block over the surface it is conforming to the highs and lows and not "Blocking" anything!

Like I said, I have tried the curvable blocks I have tried just about everything and never been happy with the performance of simply treating it like a "curved flat" panel. Goofy I know, but that is how I do it.

Brian
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Old 12-29-2009, 07:19 AM
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Quote:
Originally Posted by cutterbond
Is that Nason the yellow primer?
Because I have a friend that uses that, and I was pretty impressed by how fast you could sand it and not have it ball up the paper, and it had a nice fill to it to.

I was thinking about switching to it, because K36 takes so long to cure.
It's gray. SelectPrime 421-19. I thought it sprayed very inconsistently and seems to have created an inconsistent finish. In fact, I had to strip it entirely off some areas and am redoing them.

If I had it to do over again, I'd have just used the SlickSand polyester filler--consistent, good fill. A little orange peel, but that's easy to knock down with a D/A before blocking.
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