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Wet Sanding 101
S
This will be a comprehensive rundown because doing it
wrong can easily cost you hundreds to thousands of
dollars, so...let's get it right. This is written for
people attempting to wet sand oem paint for scratches,
etching, blemishes etc. and also body shop paint for
excessive orange peel or surface debris. Wet sanding can
give a car a "show quality" paint finish, and remove
many surface issues, but it does take some knowledge and
experience to get there.
First things first; whether you have clear coat or
single stage paint on your car (90 % of today's oem
paints are clear coat) the amount of paint is generally
about the same at 1.7 to 2 MILS. 1 MIL is a THOUSANDTH
OF AN INCH. To understand how thin this is, think about
a clear cigarette wrapper, then fold the wrapper over
itself (doubled ) THIS IS YOUR PAINT THICKNESS if you
have oem paint. another way to view it is that it's as
thin as a baby's hair (50 microns, which equals 2 MILS)
This DOES NOT include E coat/primer etc, only the TOP
LAYER of paint. Total film build with E-Coat and primer,
base color and clear coat is between 4.5 MILS and 6 MILS
generally speaking. Differences depend on make and paint
type.
Body shops GENERALLY lay it on a bit thicker because of
atomizing reasons at the gun etc. even if they only
apply 2 coats (standard) , but you can expect about 2
MILS from them unless you have asked for more coats or
been told otherwise.
Why know this? Well...wet sanding removes a LOT of paint
even at 2000 grit, so you have to have a bit of an idea
just what you have to work with.
The basic premise in wet sanding a body shop job is to
remove ONLY the top paint that has dirt/debris and
orange peel and overspray from the paint shop, and get a
glassy smooth surface. Ok, here's how you do it;
You'll need;
1).A VERY CLEAN bucket.
2). 2000 grit wet sandpaper (3M recommended) 1500 grit
is good also , but for a novice I feel 2000 is more
user-friendly since it removes paint slower.
3) A foam sanding block and a rubber sanding block. Just
get foam if you only get one, but buy both if possible.
4).3M Fine-Cut rubbing compound if by hand (or 3M
Perfect-It III Rubbing Compound if you use a rotary+
cutting pad) This is what I use, however-there are MANY
product recipes that will get you to the same result.
5).3M Finesse-It II Finishing Material (or equivalent
polish) if by hand (or 3M Machine Glaze if using a
rotary+ finishing pad)
6). 3M Imperial Hand Glaze... If you're applying a WAX
afterwards that is. This is optional as it just gets
washed away in rain or a carwash, but it will
temporarily add depth and gloss. If using a paint
sealant/ polymer, skip this step due to bonding issues.
7). PATIENCE!
Take your CLEAN bucket, add a DROP of dishwashing soap
and fill the bucket with water. Drop a few pieces of
UN-CUT wet sandpaper into the bucket. LET THEM SOAK AT
LEAST HALF AN HOUR. This is IMPERATIVE , to soften the
paper, or else you will get gouging and cutting from
sharp edges or wrinkling.
After the paper has soaked , remove it and place it face
down on a flat clean surface, and place the sanding
block in the middle of it. Now, you want to FOLD the
EDGES of the sandpaper over itself (overlap the paper)
so that there are NO FACTORY EDGES LEFT on the
sandpaper. AGAIN-THIS IS AN IMPERATIVE . You WILL get
DEEP side cuts from a factory edge that will be
IMPOSSIBLE TO REMOVE without repainting. Skip this step
and you've wasted your time.
After you've wrapped the paper around the sanding block
so there are no factory edges on it, you're ready. Use
the foam block with wet paper for areas that have curves
and are rounded, use the rubber block for the flat
areas. The foam will suffice for all if necessary. The
rubber blocks are more rigid so they actually will cut
faster too, so be aware of that.
Next, find the ridges and edges and creases and do this;
use masking tape to cover them. Sprayed paint is
affected by gravity and the film build is always thinner
on these areas, so you can't sand them down to the same
extent as other areas, and the tape will prevent you
from going through.You can always remove it after you're
finished sanding the panel and make a light pass over it
to blend it with the rest, but if you buff it out
properly you'll never notice a line.
Throw some water from the bucket on the panel and work
the paper WITH LIGHT PRESSURE in an X pattern,
crisscrossing over at each pass. This is to ensure a
flat and even result in paint removal. DO NOT SAND IN
CIRCLES, as you risk getting a wrinkle in the paper that
can cause a deep scratch. Every couple passes you want
to dunk the paper back in the bucket to rinse and re-wet
it (keep the water slightly sudsy to prevent "sticking"
or binding of the block ). Some people like to let a
garden hose run slowly over the panel, and I've done
this too, but you will have to tape the metal hose end
or it will scratch up the paint. Make sure the hose has
no dirt/sand on it either .
Every few directional passes, STOP, wipe the panel with
a cotton towel or clean rubber squeegee, allow it to
flash dry so you can see what you've got. It will look
dull and hazy, but you will be able to see where you've
taken off the surface orange peel because orange peel
will appear as darker spots in the hazy paint... WHEN
YOU HAVE SUCCESSFULLY REMOVED THE ORANGE PEEL-STOP! Some
may want to stop JUST BEFORE the orange peel is
flattened out, because it provides more "headroom " for
removing future scratches, and also gives an oem look,
which you may or may not want.
Do a panel in this manner SLOWLY and dry it to make
certain you have sanded EVENLY. You don't want to have
some areas where the orange peel is all flattened and
gone , and others where it's greatly visible. Err on the
side of caution. You can actually COUNT your passes in
the X pattern as you move along , and that might help
give you uniform paint removal. KEEP THE PAPER/BLOCK
WET!
The paper loses it's bite after a panel or so, which
then requires more pressure or passes to take the same
amount off as initially, so change the paper for each
panel at least. Make sure the rest of the paper is
SOAKING before you use it. Dump the bucket at each panel
too, so any grit/ dirt dosen't get back on the paint. 1
small grit of sand under the block can ruin an otherwise
perfect finish.
There will be areas where the X pattern will be
difficult to impossible, but just try a perpendicular
approach of overlapping. NEVER wet sand with only your
hand as a backer in 1 spot. You WILL actually see
indents from the pressure points caused by your fingers,
especially on dark colors. You CAN use your hand in
curvy areas that the blocks can't get well, but ALWAYS
move it around and overlap, and don't stay in 1 spot for
more than a couple passes.
FOR SCRATCHES ONLY: If you're only attempting to sand
out a scratch, there are a few things to know. A general
rule of thumb is; If you can feel the scratch with your
fingernail, it's probably too deep. Also- on clear
coated cars (this won't work with a white car) the
scratch MAY be removable if the scratch appears WHITE.
This is because urethane clear coat turns white when
it's abraded. If you see the car color INSIDE the
scratch, AND/OR as stated you can feel the scratch with
your fingernail as you rake across it (your fingernail
"catches" on it ) then the scratch is either too deep
with little to no paint underneath it, or through the
clear completely, and the color you're seeing in the
scratch is either basecoat or primer. Touch up or
repaint is the only alternative. Now, if you attempt to
sand out a do-able scratch, follow all the above wet
sanding instructions, BUT, sand primarily in the
DIRECTION ( not perpendicular) of the scratch. You will
have to sand the surrounding area a bit too, or you will
find you have a very flat, glassy area with no orange
peel where the scratch WAS, and the rest will have
orange peel and the sanded area may look conspicuous.
The rule here is; once the orange peel is gone, and the
scratch is still visible...you're on dangerous ground.
The more inclined you are to keep sanding to remove the
scratch fully can either make you a hero or bite you bad
if you go too far. You'll know when you sand
through-there'll be a different colored area start to
appear because the basecoat tone appears a bit different
in contrast to a basecoat with clear coat over it. (On
single stage paint you'll see primer when you've sanded
through ).You have to decide ahead of time if the
scratch bothers you enough to risk it. If you can't live
with a touch-up mark on a scratch, then go for it, but
you have to accept repainting the panel if you fail,
because the sand-through mark will be worse than the
scratch touch up in most cases. At least as obvious. A
good wet sander can save themselves or others hundreds
of dollars for a repaint in removing some scratches ,
including many on panels that have been "keyed'.
Usually, in bad cases , you have nothing to lose in
trying.
After you've done a panel to your satisfaction, you need
to buff it out with the listed compounds. By hand, you
may need to work the rubbing compound 2 maybe 3 times on
the panel before you go to the polishing (Finesse-It II
) compound. Same for the polishing compound. Use only
COTTON rags or towels if doing by hand.
By rotary, use the other compounds and the appropriate
pads. Mirror- like results await you if you have been
careful and patient.
This is something that just takes experience. It is TIME
CONSUMING, so be patient. I do recommend you practice on
a junkyard panel or a lawn mower etc. first. Better safe
and it will give you some confidence. I had to make
mistakes before I learned what not to do, and got to the
point I have with my SC and a few show quality trucks
that looked like mirrors, with no mistakes.
A word to anyone wanting to try to completely level all
the orange peel on oem paint and make it show quality;
Don't even try it unless you've done it before, and know
what you're doing. oem's are REAL thin, and you can cost
yourself a repaint fast. You get sometimes only 8-10
passes of the paper ( 1500-2000 grit) and that's all you
really get to remove before you're on shaky ground.
There is also a rule that states you should never remove
more than 3/10ths of a MIL (varies per manufacturer)
when wet sanding or buffing, due to the fact that most
of the UV inhibitors migrate to the upper .5 mil of
paint on the surface. Taking most of that UV out of the
paint leads to premature paint resin failure, at least
on cars that regularly sit in the sun. This is one
reason show cars can get away with it easier, they're
typically garage queens. If you want to know what
"premature paint failure" looks like, well, if you've
ever seen a car that had white spotting (blotches) in
the clear, you've witnessed it. Once the paint gets too
thin the UV is gone and the sun and elements break down
the resins.
In closing, this is a very valuable skill that can work
wonders on automotive paint, and solve a LOT of
problems. But it takes a few tries to get the hang of
it, and you should be ready to make mistakes, so realize
beforehand what you want to risk and what you don't.
Remember; The best rule of thumb in wet sanding = once
the orange peel is leveled, you must stop.
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