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Old 07-31-2012, 05:22 PM
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sandscratch swelling/ buffing

ok, this might be a little long winded....
had this happen on two cars...black and dark green, bc/cc, omni products.and featherfill...

the issue occurred while buffing, the paint was sanded smooth,finished to 1500 grit.

1st pass with compound, white pad,about 2000 rpm.
paint slicked right up, but then noticed what looked like scratches, only a couple, if there ws any bondo at all in this spot it was very thin, maybe a hail dent......car was d/a stripped, epoxied, rage in a couple spots, and then a coat of featherfill. blocked with 80 lightly, then 180, d/a to 220, then urethane primer....then wet sanded to 400-600. car had days in between, coats. when color sanded, it was opened and sat in the sun a couple days.

so...I sanded this area, buffed again, it was slick, then about 30 minutes later...scratches!.
so,,,,it appears that the scratch (swelling) happens when i put the buffing heat on that spot,,,it swells, looks like a scratch, I sand,,,(which is actually sandin a swell) then after buffing it shrinks and makes an actual low scratch..
so then...I sanded the area smooth again, used 3000 grit, and lightly buffed with black pad..this time got NO swelling...so its just a matter of pulling it out into the sun and seeing if it will swell and show scratches.

finally...the questions,,,

whats the deal? I dont think it's trapped solvents, I dont think its air in the bottom of a scratch, I think its 2 different materials expanding at different rates...if it is that what's the fix?
SHINE...before you tell me to lose the urethane and featherfill, know that I already have comitted to epoxy only on the truck I'm doing! lol!
but, even if I used epoxy instead of urethane, wouldnt I see the same problem?

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Old 07-31-2012, 07:45 PM
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not sure what it is but if I saw it I could tell you. Guess that doesn't help but maybe you should put it out in the sun first.
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Old 08-01-2012, 08:18 AM
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It is only one of two things and has nothing to do with expanding at different rates.

If you can wet-sand and buff out the scratch, then is is bad buffing, I won't go there, because what you are explaining is a SOLVENT trapage problem in the first coat of base.

First of all the car needs sun for a full day:
If you cannot buff out the scratches, then what it is you trapped solvents in the first coat of base, by either using too fast a reducer for the temp at hand or you just plain put the second coat of base on too fast, the solvent is setting there trapped and as you heat it, it expands and tries to escape and because paint is not airtight it does move and then you are left with a scratch showing.

It needs sun ASAP as the longer the solvent sets there it can swell a 600 grit scratch into looking like you painted over a 80 grit scratch and you further increase the chance of the solvent breaking adhesion between the clear and base.
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Old 08-01-2012, 09:03 AM
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"Opening" it up by wet sanding it THEN leaving it out in the sun will be even better.

Brian
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Old 08-01-2012, 09:59 AM
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Originally Posted by MARTINSR View Post
"Opening" it up by wet sanding it THEN leaving it out in the sun will be even better.

Brian

100% correct! Sorry I missed that.
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Old 08-01-2012, 11:57 AM
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Also keep in mind that most compounds have glaze in them and will hide
scratches. I find if I wash the panel real good after buffing I can usually
find scratches I've missed.
It's good practice to wash between buffing steps for that reason.
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Old 08-01-2012, 12:34 PM
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To elaborate on what Jim said, the glaze acts like a "clear" over it burying the scratch so you end up seeing it when that glaze wears away. This is why guys will cut and buff a car, deliver it to the customer and then the customer comes back with swirl marks and crap all over it. The glaze was like a coat of clear over the panels filling those flaws.

Brian
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Old 08-01-2012, 12:53 PM
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on the other hand, hacks love the polymer lube cause they can hide not only swirls but sand scratches upon delivery.


we just compound it, send it to get other work done, then finish it off. after 6 months we do a once over for free.
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Old 08-02-2012, 09:28 AM
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thanks for all replies, I may just be getting wayyy too picky...actually I did open it and let it sit in the sun last week, but, I'll let it sit all day today and see...the one spot is pretty much invisible now, cant even see it outside, but it is visible in the shop under lights..
Barry, did you mean I used too slow a reducer in the base(not fast)?
used dt870 thinned the paint 50% temp was 85 in the shop...waited about 60 minutes after 2nd base coat to do clear.. 2 coats base, 2 coats clear...the trapped solvent makes sense...I sure would have thought I waited long enough...well, I'll let you know...

I also sprayed one coat of sealer..mp180, i think...would have thought that would have leveled scratches a little more... would reg. epoxy have been better as a sealer?

Last edited by skip99; 08-02-2012 at 09:43 AM.
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Old 08-02-2012, 09:41 AM
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Trapping solvents that could cause the swelling/shrinking can be done many different ways. Too slow a reducer, too heavy a coat, too little flash, not atomizing properly, or even too fast a reducer with too heavy a coat! It can be a combination of many. It can also be that too much was asked of the urethane primer like filling some of those 80 grit scratches that weren't completely sanded out with the 180. There are many different factors that could be at play here.

Brian
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Old 08-02-2012, 09:42 AM
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Quote:
Originally Posted by skip99 View Post
thanks for all replies, I may just be getting wayyy too picky...actually I did open it and let it sit in the sun last week, but, I'll let it sit all day today and see...the one spot is pretty much invisible now, cant even see it outside, but it is visible in the shop under lights..
Barry, did you mean I used too slow a reducer in the base(not fast)?
used dt870 thinned the paint 50% temp was 85 in the shop...waited about 60 minutes after 2nd base coat to do clear.. 2 coats base, 2 coats clear...the trapped solvent makes sense...I sure would have thought I waited long enough...well, I'll let you know...
That is your problem, you used to fast of reducer for the temp, a 70 degree reducer at 85 is a sure way to trap solvents.
Sounds like you my have gotten off easy, as a lot of other things could have happened, not counting potential DE-lamination down the road.
Fast is not faster when used at wrong temp.
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Old 08-02-2012, 10:13 AM
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I must be having a slow day....how does using a fast reducer retain solvents? wont the 870 evap. out quicker at 85 than at 70? or does the say,,,885 keep the base wetter longer to not skin over and let solvents out?

I've just never had a problem,,,spray 870 up to about 85 degrees and use 885 if it was real hot..

get technical,,does it have to do with tail thinners and such?

you think i would have been better off using 885, ?
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Old 08-02-2012, 12:55 PM
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80-85 is the hottest you can spray a regular reducer. It will skin over due to heat and trap solvents.
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Old 08-02-2012, 01:09 PM
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Like 69 said but you also had two other issues going against you.
Dark green and Black, these are two of the three slowest drying colors because of the carbon in them.
You might have gotten away with this if car was white, silver baby blue, yellow or red.
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Old 08-02-2012, 01:27 PM
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Quote:
Originally Posted by skip99 View Post
I must be having a slow day....how does using a fast reducer retain solvents? wont the 870 evap. out quicker at 85 than at 70? or does the say,,,885 keep the base wetter longer to not skin over and let solvents out?

I've just never had a problem,,,spray 870 up to about 85 degrees and use 885 if it was real hot..

get technical,,does it have to do with tail thinners and such?

you think i would have been better off using 885, ?
I can't get "technical" I can only say that when you pile on a number of coats without proper flash time, solvents can get trapped. And in fact when it's too fast it can do it worse than if it's too slow! Let's say you using the fastest reducer designed for 50 degrees. You are spraying this paint in 100 degree weather. You fill up your gun and you bomb on three coats one after another very heavy trying to keep it wet because it's flashing so fast. The top layer of the film CAN flash off and does, creating a surface that is now closed up so the solvents under it can't get out. You feel it, it feels "dry" and you put clear over it. Now the clear seals that all off. You have solvent trapped in the base, that solvent is keeping isos from linking up, it isn't "cured" and is soft. It begins to move around, sink, etc.

That is why there are many ways it can happen. Simply applying it "correctly" with the wrong reducer or using the right reducer and applying it wrong can produce the same trapped solvents. Just as Tech said, then you have the color issue as Barry said, there are many other factors. I call it when the planets are aligned, it's a perfect storm. Sorry about the double analogy.

Brian
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