deep scratches correction- question

pampos · 05-23-08, 08:12

i am wondering if there any scratches that cannot be removed by rotary polisher or wetsanding...
of course i am working with a DA polisher but i really want to know...
when someone should stop trying to remove a scratch before the CC or paint has gone??

Accumulator · 05-23-08, 08:24

Yeah, there are plenty of scratches that are too deep to be removed, and (noting that this is in the Pro Forum) it's something to keep in mind as messing up your own personal car is one thing, but messing up a *customer's* car is something else!

Generally, you shouldn't remove more than 0.3-0.5 of a mil (that's .003"-.005") without precipitating premature clearcoat failure. That means that many/most scratches that'll catch your fingernail are at least *close* to being too deep for complete removal.

On single stage, IME you can thin the paint more (assuming it's thick enough that you don't hit primer), but eventually it'll often catch up with you as ss usually oxidizes quite readily and subsequent work to remove that will usually take off even more paint. I've gotten by for years with *very* thin ss, but only on pampered vehicles. Also, some ss enamels don't respond well to aggressive correction (the uppermost build-film needs to be left basically intact- search for Guitar Man's old posts on this for more info).

To determine how much paint you're taking off, you need an ETG and some kind of baseline (problem: how to figure out how much clear was removed in the past

).

And note that when it comes to serious correction of this nature, wetsanding followed by rotary work will take off less clear than prolonged work with a random orbital *IF* (big "if") you do the wetsanding/rotary work properly.

If in doubt, don't polish it out

pampos · 05-23-08, 08:53

Quote:

Originally Posted by Accumulator

Yeah, there are plenty of scratches that are too deep to be removed, and (noting that this is in the Pro Forum) it's something to keep in mind as messing up your own personal car is one thing, but messing up a *customer's* car is something else!

Generally, you shouldn't remove more than 0.3-0.5 of a mil (that's .003"-.005") without precipitating premature clearcoat failure. That means that many/most scratches that'll catch your fingernail are at least *close* to being too deep for complete removal.

On single stage, IME you can thin the paint more (assuming it's thick enough that you don't hit primer), but eventually it'll often catch up with you as ss usually oxidizes quite readily and subsequent work to remove that will usually take off even more paint. I've gotten by for years with *very* thin ss, but only on pampered vehicles. Also, some ss enamels don't respond well to aggressive correction (the uppermost build-film needs to be left basically intact- search for Guitar Man's old posts on this for more info).

To determine how much paint you're taking off, you need an ETG and some kind of baseline (problem: how to figure out how much clear was removed in the past

).

And note that when it comes to serious correction of this nature, wetsanding followed by rotary work will take off less clear than prolonged work with a random orbital *IF* (big "if") you do the wetsanding/rotary work properly.

If in doubt, don't polish it out

wow that's science

so in simple words if i can feel the scratch it is better not try to remove it in anyway even if the primer doesn't shown yet??
because i am working with UDM i don't know when a scratch it is too deep to remove or when my UDM has not the cutting power to remove it

.Is there a way to separate those two??

Accumulator · 05-23-08, 09:30

Quote:

Originally Posted by pampos

so in simple words if i can feel the scratch it is better not try to remove it in anyway even if the primer doesn't shown yet??

Yeah, but that's just *IMO*. Others get very aggressive and say it's OK so it's a question of how lucky you feel.

With ss, you don't want the primer to show and if the colorcoat gets thin enough the primer will start to show through. With basecoat/clearcoat you need to leave a lot of clear on top of the colorcoat as the paint will fail if you take off just a *little* too much clear- it'll look fine until it's spent some time in the sun and they you'll either get clearcoat failure or the basecoat will start to fade under the clear. In either case the only fix is a repaint.

Quote:

ecause i am working with UDM i don't know when a scratch it is too deep to remove or when my UDM has not the cutting power to remove it

.Is there a way to separate those two??

The UDM oughta have the power to remove any marring *IF* (big "if"

) you work it long enough; I've removed marring from hard Audi clear (behind door handles) by *hand* (yeah, it took *forever*). It's simply a matter of "sanding away the paint" with the machine/pad/polish until you get to the bottom of the scratch's "valley", then things look level so you don't see the scratch any more.

So yeah, there's a big difference between what your UDM *can* do and what you *should* do with it- if you work a spot long enough you can take off all the paint

Fortunately, doing major correction with machines like the UDM takes *so* long that you're likely to say "eh, forget it...good enough" long before you do much damage (at least if you're like *I* am

).

FWIW, I've killed clearcoat with a PC using 4" pads. I wasn't being all *that* aggressive, but I cut completely through the clear to the colorcoat. The only consolation was that the scratch I was working was so deep I knew the panel would have to be repainted anyhow.

Shooting for perfection can be a tricky thing if the paint is really marred up...taking off a lot of clear/paint is simply risky when it comes to the paint's long-term durability.

toyotaguy · 05-23-08, 10:08

general rule of thumb is that if you can feel and catch it with your fingernail, its NOT coming out, but you can add paint and then wetsand and compound/polish it smooth...

dublifecrisis · 05-23-08, 11:26

good stuff. all of you! it's great just to sit and read sometimes.

salty · 05-23-08, 11:22

Occasionally the scratch will be too fine to catch your nail but be too deep to remove.

pampos · 05-24-08, 03:19

And according all of these when i have to stop trying??

toyotaguy · 05-24-08, 11:08

paint thickness gauge will tell you that one....some cars you can work 10 tens with a heavy compound, some you have to hit it twice and be careful from then on...

Accumulator · 05-24-08, 11:09

Quote:

Originally Posted by salty

Occasionally the scratch will be too fine to catch your nail but be too deep to remove.

Heh heh, that's what I think too, but some people would just wetsand it out.

Quote:

Originally Posted by toyotaguy

.. if you can feel and catch it with your fingernail, its NOT coming out, but you can add paint and then wetsand and compound/polish it smooth...

I've done that on some of the M3's nastier scratches (just needed clear, not basecoat). Doesn't turn out 100% for me, but it's still better than nothing. And somebody else might get much better results than I do.

Quote:

Originally Posted by pampos

And according all of these when i have to stop trying??

When your ETG says the paint is getting thin (IIRC, a *VERY* rough rule of thumb is that ~80-100 microns is thin enough that you need to really think about it). I gotta play around some more with mine before I have a clear idea of what's "thin".

salty · 05-24-08, 11:37

I'm just leaving to go and try to fix a dark blue Alero which the bra has being removed. I do not plan to sand all the damage out, wish i had a PTG. If it appears 50% or better i think she will be happy.

pampos · 05-24-08, 07:58

Quote:

Originally Posted by salty

I'm just leaving to go and try to fix a dark blue Alero which the bra has being removed. I do not plan to sand all the damage out, wish i had a PTG. If it appears 50% or better i think she will be happy.

i do the same thing explaining to people that some scratches are very deep to be removed and they will have problems next time they will polish the car IF we remove too much clear coat at this time. is that right or not?i don't have a PTG at the moment : ( and almost all the cars i detailed until now they had a lot of deep scratches and most of them seems that they have been heavy polished before (as much as i understand).
I know that i must have a PTG and i am going to buy one as soon as i can, but what i have to do until then??
by wet sanding a scratch there is no danger to remove too much clear coat until the scratch is gone??