apparent color change after sanding.

Discuss anything after that final masking comes off.
PostPosted: Sat Sep 25, 2004 8:52 am
well.. after spraying my hood the final coat of spray, I wet sanded with 2000 grit, then used 3m perfect it II rubbing compound, then polished it. Its nice and glossy, and other than a few flaws due to bad enviroment (my garage, I didnt pay enough attention to cleanliness there apparently
:( ) it looks decent for a first paint job. However, in the right light it looks almost a dark grey (car is black) it matched VERY well before the sanding and buffing, but not so well now.. any ideas? No telling what I messed up hehe, but.. if it can be messed up, Im sure I can do it :P.. thanks in advance for the help.

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PostPosted: Sat Sep 25, 2004 1:43 pm
Are you sure you did your final polishing properly? A rough surface can turn black a bit light. Could be it's a bit of blushing too, generally caused by humidity, not sure how you can fix that now besides a respray. Could be dieback in the clear too, what kind of paint did you use and what was the humidity like at the time in the air? Also what does your air supply look like?
PostPosted: Sat Sep 25, 2004 2:46 pm
btw that was me.. for some reason I cant seem to stay logged in... ahh well, Ill figure that one out later.. on to the topic at hand.

paint was ppg omni line, as was the clear coat.. air supply was a compressor (15.9 scfm at 40 ((I shot at 50, conventional gun)) run down about 75 feet of PVC to a filter/dryer, then through a 50' length of hose to the gun. wasnt real humid at the time, prolly around 40ish percent or so according to the weather channel, cool day for this area, was about 70ish.

As for polishing, well.. I followed the advice of our local supply store. It looked good as far as color until I sanded it, then put it back on the car, now it doesnt match :(... Maybe I am not polising enough?

Used 3m wool compounding pad, with perfect it 2 compound, then went to a foam polishing pad and perfect it 3 machine glaze. Speed about 1000rpm. Didnt use much pressure, and didnt buff for a long time as I was afraid to burn it..

thanks again for the help so far, hoping I can get this resolved or at very least a definitive answer on what went wrong so i can NOT do it again... having a BLAST learning though, never though painting/body work could be this much fun...

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PostPosted: Sat Sep 25, 2004 3:52 pm
Ok, first off you need a real air supply. This is the reason the industry is pushing out amateurs, because modern society feels the need to protect people from their own stupidity. Personally I think that natural selection has worked for thousands of years so why **** with it now, but I guess not many people think like I do. Compressed gas in PVC is VERY dangerous. Even schedule 8, or whatever the strongest is, isn't designed for compressed gas. You might never see it explode, but if it does it'll be the last thing you see. It doesn't rupture like a metal line, it just explodes into tiny plastic shrapnel. Do yourself and anyone that might enter your shop a favour and spend $50 on a real air hose. I'm not trying to call you down, everyone makes mistakes, and it's hard when theres all kinds of morons and halfwits out there that will try to convince you PVC is safe for compressed air. My apologies if the PVC is buried underground or something. Some people do that and it seems safe, even if not smart, but I know a lot will run PVC through the middle of their work space and think nothing of it.

I doubt it's blush becuase it was cool and not humid. It could be dieback... I'm not even sure of all the causes but I know there are several. It could also be you're being stingy with the machine glaze, give an area a good working with the buffer to find out. Bump up the speed to 1500rpm. As long as you have 3 coats of clear a burn through is unlikely unless you sanded quite a lot.

Maybe someone else can speak up about dieback or something else that could cause it, I'm not sure what else it could be.

And yeah, painting is fun, especially if it's your own stuff. You have to go through what you're dealing with right now, but just pay attention and make sure you learn from it. Good luck!



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PostPosted: Sat Sep 25, 2004 5:07 pm
kk.. got the log in fixed..

yeah I am aware NOW of the problems with PVC, was told it was safe by several people I know using it, then AFTER I plumbed it and ran it, found out from someone else I know that not only it is it inferior to copper, but very unsafe.. I supose I should have figured it out on my own, but.. hindsight is so much better than foresight. However, we are moving soon possibly so, no copper until after the relocation.

anyways. thanks for the tips, Ill try more buffing and glaze, see if that helps.. thanks Aberrant, preciate the help. Ive already learned ALOT from this experience, at this rate I should be **** near perfect by the next thing I paint :P...

Love doing my own stuff, even if it isnt perfect, it is so rewarding...



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PostPosted: Sun Sep 26, 2004 11:46 am
I agree PVC as air lines not good. The PSI rating on the pipe is for liquid not gases. I've heard of alot of people getting hurt using the pipe for potato guns.
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PostPosted: Sat Oct 30, 2004 6:36 pm
i would use black steel pipe or galvinized instead of copper. copper is mucho expensive, and i don;t know what the pressure rating is but i would think it is plenty. black pipe is higher since it is made for gas, and i often see and use galvinized it works fine for most general uses.

does it need to be cleaned for paint use say you install new compressor and pipes do you flush to clean oils out of it?
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PostPosted: Mon Nov 01, 2004 3:39 am
Jeffw, most of the black pipe you see used for gas is natural gas running at pressures so low, they measure in inches of water column rather than psi. The pipe is more for protection than pressure. I wouldn't think about black pipe for an air line because - IT RUSTS. Galvanized is fine but pretty hard to work with compared to copper. Several guys I read recently say their copper air system was less than 10 % more than iron pipe and a lot easier to install. I use copper pipe in residential A/C systems where the working pressure can be close to 400 psi, both in liquid and gas form. Even if you hard solder the joints its a lot easier than cutting and threading pipe.

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