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Sanding compound curves...

21K views 32 replies 9 participants last post by  krazeyone 
#1 ·
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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#3 ·
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...
 
#5 ·
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
 
#7 ·
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... :D
 
#9 ·
wyomingclimber said:
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... :D
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.
 
#10 ·
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.
 
#12 ·
cutterbond said:
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. :mad:
 
#13 ·
wyomingclimber said:
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. :mad:
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.
 
#14 ·
302 Z28 said:
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
 
#15 ·
cutterbond said:
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.
 
#17 ·
Nason 2k is a pretty good primer ,can you dent the primer with your fingernail? if its balling up it might not have enough hardner in it.try wiping lacquer thinner on it if it comes off, thats the problem ,if its cured it wont bother it at all....if its not cured you got to get it off and redo it.Its a beech ,I know first hand but its the only way........I learned this trick from an old timer it works great for curves.....Take a full sleeve of wet sand paper and get it very wet in a bucket of water......fold the whole wad in half or thirds down the long way,it'll be about an inch thick,the paper itself is the block, start wet sanding ...it'll be stiff enough and flexable enough to do a great job.sand in a circular motion (just on the curves) then as the paper wears down just peel a piece off and keep going....this is also a great way to sand clear....
 
#18 ·
deadbodyman said:
Nason 2k is a pretty good primer ,can you dent the primer with your fingernail? if its balling up it might not have enough hardner in it.try wiping lacquer thinner on it if it comes off, thats the problem ,if its cured it wont bother it at all....if its not cured you got to get it off and redo it.Its a beech ,I know first hand but its the only way........I learned this trick from an old timer it works great for curves.....Take a full sleeve of wet sand paper and get it very wet in a bucket of water......fold the whole wad in half or thirds down the long way,it'll be about an inch thick,the paper itself is the block, start wet sanding ...it'll be stiff enough and flexable enough to do a great job.sand in a circular motion (just on the curves) then as the paper wears down just peel a piece off and keep going....this is also a great way to sand clear....
Thanks for the tips on testing the paint--it's really been weighing on my mind. Can't dent it with my nail. I'm going to go out and buy some laquer thinner later and run that test too.

If I have to take it off and redo it, I may just lay under the tires and have my wife run me over a few times instead. :rolleyes:

So many interesting ideas for working those curves--I'm looking forward to trying them. That area of the truck is right at eye level and of course it's the hardest place to get right. Murphy's law...
 
#19 ·
wyomingclimber said:
Thanks for the tips on testing the paint--it's really been weighing on my mind. Can't dent it with my nail. I'm going to go out and buy some laquer thinner later and run that test too.

If I have to take it off and redo it, I may just lay under the tires and have my wife run me over a few times instead. :rolleyes:

So many interesting ideas for working those curves--I'm looking forward to trying them. That area of the truck is right at eye level and of course it's the hardest place to get right. Murphy's law...
I have to warn you though, not all 2K primer is created equal! There are 2K products out there that even when mixed properly AREN'T insoluable to lacquer thinner! So just because it may soften up with the lacquer thinner rub test it doesn't mean that it isn't "performing". It may not be performing as well as some other primer, but it maybe doing just as the manufacturer expected.

Brian
 
#20 ·
Nason 2k is the primer I was talking about. I forgot to catalyze a 2k primer twice when feather fill first came out I was used to lacquer primer (industry standard) at the time.so i know your pain. I wouldn't think its the problem if its not soft. BUT.. I am the local expert around here at removing un catalyzed primers and paints :mwink:(the last time I mixed clear and wasnt paying attention I mixed a 1:1 clear at 4;1) and pretty much fixing any kind of screw up...every time I think I've screwed everything up there is and cant possibly screw anything else up ,I amaze myself....seriously though If you need to get it off I can help make it easier,lets hope thats not the case.Can you gouge the primer out with your finger nail?
 
#21 ·
deadbodyman said:
Can you gouge the primer out with your finger nail?
No way. I pushed my thumb nail as hard as I could into it and it didn't make a mark I could feel running my finger over it. Gonna try the lacquer test tomorrow.

I honestly don't know why there would be a problem. I'm sure I catalyzed it. In fact, my problem seemed to be the opposite. I had to strip it all off the box because of dry spray.
 
#22 · (Edited)
It sounds like its hardened to me.I dont know why it would roll and clog the paper,unless you used the wrong reducer or hardner by mistake. I've done that too,I had to check all the levels of all my hardners. What grit paper are you sanding with? wet or dry?do you have any other paint materials on the same bench as the primer? One more thing... 180 and finer grits get packed with powder pretty quick, You have to slap the pad with a paint paddle or something to loosen it up and clean the paper when you dry sand , wet sanding works better. in my opinion. also if you lay down your guide coat to thick it;ll roll too but its the guide coat rolling not the primer.If it sounds like I've made a lot of mistakes in my life as a body & paint man, your right I have, No need for you to too.try a few of these ideas and let us know.I cant have made every screw up..but these are some of the ones I remember.
 
#23 ·
deadbodyman said:
180 and finer grits get packed with powder pretty quick, You have to slap the pad with a paint paddle or something to loosen it up and clean the paper when you dry sand.
Maybe that's my problem--I'm using 220. It gets packed up in a solid mass and I have to hit the sandpaper with the compressor to get it off. I'm sure I used the right hardener--I'm way too absent minded to allow anything but the stuff I'm currently using to be anywhere within my line of sight.

deadbodyman said:
If it sounds like I've made a lot of mistakes in my life as a body & paint man, your right I have, No need for you to too.try a few of these ideas and let us know.I cant have made every screw up..but these are some of the ones I remember.
I hear you. I figure if I did this again, I could do it in 1/3 the time for 1/2 the money. I didn't even know how to check my oil when I started this project. If it wasn't for this board, I'd still be staring at a 10 foot high pile of rusted parts.

I'm looking forward to trying all the techniques suggest. I'm praying I don't have to resort to your wet sanding idea, though. I think there would be ice floating in my bucket this morning...
 
#24 ·
IF all else fails, an old pirate's trick:

(you will need 6" stick-it DA paper, Saran Wrap, spray-can Styrofoam home insulation)

1. cut 8 1.5" pie shaped slices around the paper, should leave a 3" sun and 8 rays, save the clippings
2. place paper over a coffee cup of hot water, the stream will let the paper go limp
3. place paper, grit up, where you wish to work, start at center and layout flat, install clippings into slots. Make it pretty, style points count :thumbup:
4. cover area with Saran-Wrap, no wrinkles.
5. apply Styrofoam to cover paper size and allow to cure
6. trim sanding block to fit your hand but slightly larger than the paper
7. install new paper grit down and attach sanding block, check for perfect contact, the slots leave somewhere for the dust to flow.
8. go to town with your brand new, PERFECTLY sized and shaped sanding block. These only work for a small area thanks to constantly changing curves, you will see and fell the limit instantly.

Guide coat is your friend, patience is a virtue and Happy New Year.
 
#25 ·
My God, that sounds harder than building the thing! :D

I found some interesting sanding pads at Ace. Three in a pack and they can be stacked kind of like your wet sandpaper concept in order to give them a little stiffness.

I think (hope) they might do the trick, working in small areas at a time w/some guidecoat.

It's just one of those scary things because you aren't really gonna know til you get the paint on it and it's too late...
 
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