Today's post is
an exercise in bad photography, so an apology up front!
BONDO - In southern America, it's
a generic term like band aids, kleenex or Coke (well, anywhere near Atlanta
anyway)... A Bondo Buggy brings a
specific vintage vehicle to mind down here.
In the larger world Bondo is known as Plastic Body Filler, Polyester
Filler, Mud, Body Butter, Wreck Concealer - it's a catalyzed polyester putty
used to fill in dents. The process
involves mixing and glopping the stuff on body panels and then sanding for
days, weeks, months. It’s conducive to
neither interesting posts nor photos, especially bad photos.
I've been doing A
LOT of it lately. So here's the Bonehead
method of “reading” bondo. We’ll use the
passenger door as our canvass; it has
wobbles along the lower edge. And per
everyone's instructions for applying mud, we’re applying it to clean unpainted,
rust-free metal. All the welding and
such is finished. Plastic filler is
great when used correctly, but it won’t make rust disappear.
Looking
and feeling will tell you a lot about a panel, but mud can give you solid
information too - if you listen to what it's telling you. I know that in the days of yore, body dudes
used lead (now days a tin alloy) to fill in “imperfections”, but the stuff is
dangerous, AND as one interwebber says, “they only used lead because they
didn’t have bondo”.
PREP: Do not put filler on dirty metal - it won't stick! Also rough up the area with some 80 grit sandpaper so that the filler has something to physically grab. DO NOT SKIMP ON THE PREP!
THE GOOP: You’ll need a high-quality filler, one that has had the air bubble vacuumed out and applies smoothly. Don’t skimp on the stuff - not the place to save buckage!
FLAT SANDING BOARDS: I’m using 16 and 11-inch boards
and the 16-inch most of the time.
Sanding with a small block, or worse sandpaper in hand, leads to wavy,
uneven surfaces.
First application of filler with a few passes with the 16" board.
The low spots are already revealing themselves.
Sweat
and achy muscles: Ain’t no way around
the sanding and power sanding takes the stuff off too fast. You gotta earn the sleek, smooth body, so git
at it!
NOT
THICK: First, clean the surface – I use lacquer
thinner. Mix up the mud according to
instructions and smooth it on. Don’t
glop a thick clumpy layer on! The more
you slap on, the more you must sand off.
The goal isn’t to get it done in one filler application.
SANDING: The magic is the sanding. Pay attention to what areas sand first, which
sand last and which areas resist “feathering.”
Read the sand down. (BTW I loaded
80 grit sandpaper to the boards.)
STROKES: Keep the sanding board oriented perpendicular to
the curve of the panel. Most of the time
that means horizontal to the panel. For
our door, I kept the board horizontal because orienting the board vertically
would create flat areas on the curve of the door.
Use
diagonal strokes while maintaining the orientation of the board. I would make downward diagonal strokes for a
while, then switch to upward diagonal strokes.
If I had kept strictly horizontal strokes, the board would not follow
the vertical curve of the door, although it would keep the horizontal line
straight.
“Feathering” happens at the
edges. As you reduce the mud to fit the
panel, the edges will begin to disappear and feather into the surrounding
metal. If an edge resists feathering,
it’s probably low. Forcing it to feather
will create a larger low spot in the filler, so stop sanding! Over sanding is a more common mistake than
under sanding.
Some graphics to show the board orientation and stroke directions
 |
| Finished sanding of the first application. Low spots and unfeathered edges are giving me info |
SHINY METAL: An area of shiny
metal probably indicates a high spot, especially if the edges are clearly
defined. Apply hand. Close your eyes and run your palm along the
area many times. If it feels high, it
will show. The filler may be too low, or
you may need to hammer down the area a bit and get it on the next application
of mud. I hammered the high spots down a bit before the next application of mud.
 |
| Second Application: The shiny spots were sanded shiny and are high spots. Low Spots are fewer but not gone. |
PRIMER: Shoot some primer over the panel and wet sand
it with 400 grit sandpaper. Look for visible
edges, high metal or other lumps or valleys that mean the filler isn’t
flat. Sanding scratches aren’t a big
concern at this point; a catalyzed thick coat primer will get most of them when
you’re closer to shooting paint.
 |
| A truly bad pic - it's looking horizontally down the door. I'm trying to show that the door is smooth even if a lot of the scratches show |
APPLICATIONS: It took four applications to get everything
filled. I prefer to put on thin coats instead
of thick ones. My theory is (and I don’t
know if the professionals back me on this) that multiple layers of thin filler
are stronger and hold up better than thick layers. Remember, I did a lot of hammer and dolly work
to keep the total filler layer as thin as possible.
MISTAKES!
OVER-SANDING: Sanding off too much filler is easier to do
than it sounds. If you find yourself
sanding to force a low spot to disappear or and edge to feather, you’re
probably over-sanding. Let the filler instruct
you. Don’t try to force it to do what
you want. Stop, close your eyes and feel.
SMALL BLOCKS: Small sanding blocks or handheld sandpaper
leads to waves. It may cut through the
filler quickly, but you’ll just have to do it again.
IMPATIENCE: Body work takes time! Trying to get things done and out of the way is the wrong mindset. Assume it will suck and take forever, 'cause it will.
PLASTIC FILLER IS NOT BODY WORK: Filler will not stick to rust! Let me repeat, filler will not stick to rust. Oh, it will for a while, probably until you have spent the money on paint, but it will fall off sooner than you think. Plastic filler DOES NOT SUBSTITUTE FOR BODY WORK. Weld in new metal and hammer and dolly first. Filler covers small imperfections, not gaping wounds
