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Bondo Basics
Bondo Basics, by Brian Martin
What ever tools you use, the trick is to not add the last "skim coat" till you KNOW that it
is all you need. Don't try to block out that first coat, just use it as a base for the LAST
skim coat. I was taught this procedure after doing bodywork for a number of years and it
really works well:
Just apply a nice coat of filler (what ever brand, whatever style, (we will put that aside
right now). Cut that coat NOT to make it perfect, but to get the basic shape and filling
you need as a base for the skim coat. You can cut it with 36, 40 or 80 depending on
how big the area you are working is. In other words, if you can cut it fast with only 80
then do it. But I would say that this would be limited to an application that is no larger
than about 8 inches.
If you happen to have a few high spots, see if you can tap them down. If you have a few
low spots add a bit more filler to ONLY those spots.
Re-cut these last low spots you have just filled with the same grit you have been using
(most likely 36).
If you now have a surface that ONE skim coat will fill, then apply it. If you don't, work with
it a bit more, but NEVER add a little here or there and think you will finish it without a
skim coat.
If you have a surface that is very close with only a few VERY MINOR low spots like poor
feathering onto the metal, poor transitions from one application of filler to another, or
from the metal that is "poking" up here and there you can do the LAST skim coat.
This skim coat is very important, you want it to extend over the COMPLETE area, this is
well past the damage you have been working. Maybe as much as 3 inches past the
plastic that you have applied to "rough" it out.
This skim coat can be regular filler or a polyester glaze like "Icing" or "Polyester glazing
putty", that is your choice, I use both depending on the size of the area being worked.
Do not use anything that doesn’t mix with a hardener. NO, “Spot putty” in a tube, only
polyester putties or fillers. If it uses a hardener, it cures to a hard film. The “spot putties”
stay soft and can become even softer when the solvent from the primer coats it.
You now run a block, long board, or hog even over this skim coat with a little bit coarser
paper than you plan on finishing with to cut off the resin that has surfaced in the filler. I
usually just use the 36 or 40 or whatever I have been on the "rough" work. BUT take
CAUTION not to cut much off, you want to JUST take the very top, don't really sand AT
ALL.
Now finish sanding with your longboard or block or hog or whatever using the finer
paper like 80 on a large area or 120 on that small 8" sized area. Block it out to
perfection with a nice feather edge to the surrounding metal.
I can't stress enough, the trick is to know when just ONE LAST skim coat will do the job.
And apply it COMPLETELY over the surface. If you only have one little low spot in the
middle, don't just do it, skim the ENTIRE thing. You must have one LAST skim coat over
the ENTIRE thing every time. If you get in the habit of this you will do it over and over on
every dent you repair and find that you can do just about any dent with just two
applications.
As you sand the filler let the board or block you are using run over the surrounding
metal. If you only work on the filler you will sand it too low. You need to keep it as high
as the surrounding metal, so use the metal as sort of a straight edge that you run the
block or board off of.
Don’t worry if you cut through this skim coat here and there. In fact, you WILL most likely
cut through. The point of that "LAST SKIM COAT" is that after you add it, you don't add
any more filler. That "LAST SKIM COAT" is just that the LAST filler you add. If you hit a
little filler below, or metal, that is normal and fine. The only thing you are looking for at
that point is if the panel is FLAT. The filler skim coat is serving no other purpose than to
finish you filler work, it is not a "sealer" or anything like that.
You can add fiberglass resin (“A” coat if you have a choice) adding the resin was exactly
how I learned from the great Emery Robinson (my personal hero in the auto body
world). But remember there was no products like polyester putties back then. When you
add resin, that resin comes to the top of the film of filler. It is then something you have to
deal with. The whole purpose of the SKIM COAT is to put a layer of filler over the top
that is easy to block out with as little effort as possible. You want to be able to
concentrate on making the panel FLAT not fighting with gummy resin, sand scratches
and the like.
So the polyester putty, though expensive, is what I use.
How is this for an idea, a co-worker of mine showed me this very obvious tip.
Add pour-able polyester putty to the regular filler! What an idea! LOL A little pour-able
squirted into the "bondo" really thins it out nicely.
The "LAST SKIM COAT" should be left to cure a good long time. Where you may jump
on filler and sand it as soon as it is hard, the skim coat should be GOOD AND CURED
for an hour or more. If you can of course, in the production shop you may not be able to
wait that long. The benefits of the procedure will not be diminished.
A little added note, I have found that I don’t use 36 or 40 grit at all anymore. I went to
work at a shop that didn’t use the coarser grits so I had to learn not to also. I have found
that using just the 80 and then finishing the Skim coat in 120 or 180 works great, even
on large panels.
At this shop it was the first time that I wasn’t doing my own primer work. This meant that I
couldn’t “cheat” with a lot of primer and blocking the body work “one more time”. I found
that I had to get the work PERFECT, then give it to the painter. I did this in an interesting
way, I look at the last skim coat as even a more “final” step. I now look at it as “primer”.
You see I have used polyester primer, which is like spraying “bondo”. They are both
polyester resin based and act and sand very much the same. So, I figured why not just
“spread out my primer” as the skim coat! It has worked GREAT, the painter jokingly
says, “do you think I’ll need to prime this or just paint it?” I tell him, “Just clear it, it’s a
shame to hide that work under primer”.
This method has worked great for me, it’s more of a state of mind than a procedure.
And don’t be afraid to buy the best sand paper and use a lot of it, the cost of the paper
will be nothing next to the time and muscles saved. Find the paint store in town that
services the PROS the Body shops in town, that is were you will get the right stuff and
the right info.
Article Courtesy of www.buicks.net
Bondo Basics, by Brian Martin
What ever tools you use, the trick is to not add the last "skim coat" till you KNOW that it
is all you need. Don't try to block out that first coat, just use it as a base for the LAST
skim coat. I was taught this procedure after doing bodywork for a number of years and it
really works well:
Just apply a nice coat of filler (what ever brand, whatever style, (we will put that aside
right now). Cut that coat NOT to make it perfect, but to get the basic shape and filling
you need as a base for the skim coat. You can cut it with 36, 40 or 80 depending on
how big the area you are working is. In other words, if you can cut it fast with only 80
then do it. But I would say that this would be limited to an application that is no larger
than about 8 inches.
If you happen to have a few high spots, see if you can tap them down. If you have a few
low spots add a bit more filler to ONLY those spots.
Re-cut these last low spots you have just filled with the same grit you have been using
(most likely 36).
If you now have a surface that ONE skim coat will fill, then apply it. If you don't, work with
it a bit more, but NEVER add a little here or there and think you will finish it without a
skim coat.
If you have a surface that is very close with only a few VERY MINOR low spots like poor
feathering onto the metal, poor transitions from one application of filler to another, or
from the metal that is "poking" up here and there you can do the LAST skim coat.
This skim coat is very important, you want it to extend over the COMPLETE area, this is
well past the damage you have been working. Maybe as much as 3 inches past the
plastic that you have applied to "rough" it out.
This skim coat can be regular filler or a polyester glaze like "Icing" or "Polyester glazing
putty", that is your choice, I use both depending on the size of the area being worked.
Do not use anything that doesn’t mix with a hardener. NO, “Spot putty” in a tube, only
polyester putties or fillers. If it uses a hardener, it cures to a hard film. The “spot putties”
stay soft and can become even softer when the solvent from the primer coats it.
You now run a block, long board, or hog even over this skim coat with a little bit coarser
paper than you plan on finishing with to cut off the resin that has surfaced in the filler. I
usually just use the 36 or 40 or whatever I have been on the "rough" work. BUT take
CAUTION not to cut much off, you want to JUST take the very top, don't really sand AT
ALL.
Now finish sanding with your longboard or block or hog or whatever using the finer
paper like 80 on a large area or 120 on that small 8" sized area. Block it out to
perfection with a nice feather edge to the surrounding metal.
I can't stress enough, the trick is to know when just ONE LAST skim coat will do the job.
And apply it COMPLETELY over the surface. If you only have one little low spot in the
middle, don't just do it, skim the ENTIRE thing. You must have one LAST skim coat over
the ENTIRE thing every time. If you get in the habit of this you will do it over and over on
every dent you repair and find that you can do just about any dent with just two
applications.
As you sand the filler let the board or block you are using run over the surrounding
metal. If you only work on the filler you will sand it too low. You need to keep it as high
as the surrounding metal, so use the metal as sort of a straight edge that you run the
block or board off of.
Don’t worry if you cut through this skim coat here and there. In fact, you WILL most likely
cut through. The point of that "LAST SKIM COAT" is that after you add it, you don't add
any more filler. That "LAST SKIM COAT" is just that the LAST filler you add. If you hit a
little filler below, or metal, that is normal and fine. The only thing you are looking for at
that point is if the panel is FLAT. The filler skim coat is serving no other purpose than to
finish you filler work, it is not a "sealer" or anything like that.
You can add fiberglass resin (“A” coat if you have a choice) adding the resin was exactly
how I learned from the great Emery Robinson (my personal hero in the auto body
world). But remember there was no products like polyester putties back then. When you
add resin, that resin comes to the top of the film of filler. It is then something you have to
deal with. The whole purpose of the SKIM COAT is to put a layer of filler over the top
that is easy to block out with as little effort as possible. You want to be able to
concentrate on making the panel FLAT not fighting with gummy resin, sand scratches
and the like.
So the polyester putty, though expensive, is what I use.
How is this for an idea, a co-worker of mine showed me this very obvious tip.
Add pour-able polyester putty to the regular filler! What an idea! LOL A little pour-able
squirted into the "bondo" really thins it out nicely.
The "LAST SKIM COAT" should be left to cure a good long time. Where you may jump
on filler and sand it as soon as it is hard, the skim coat should be GOOD AND CURED
for an hour or more. If you can of course, in the production shop you may not be able to
wait that long. The benefits of the procedure will not be diminished.
A little added note, I have found that I don’t use 36 or 40 grit at all anymore. I went to
work at a shop that didn’t use the coarser grits so I had to learn not to also. I have found
that using just the 80 and then finishing the Skim coat in 120 or 180 works great, even
on large panels.
At this shop it was the first time that I wasn’t doing my own primer work. This meant that I
couldn’t “cheat” with a lot of primer and blocking the body work “one more time”. I found
that I had to get the work PERFECT, then give it to the painter. I did this in an interesting
way, I look at the last skim coat as even a more “final” step. I now look at it as “primer”.
You see I have used polyester primer, which is like spraying “bondo”. They are both
polyester resin based and act and sand very much the same. So, I figured why not just
“spread out my primer” as the skim coat! It has worked GREAT, the painter jokingly
says, “do you think I’ll need to prime this or just paint it?” I tell him, “Just clear it, it’s a
shame to hide that work under primer”.
This method has worked great for me, it’s more of a state of mind than a procedure.
And don’t be afraid to buy the best sand paper and use a lot of it, the cost of the paper
will be nothing next to the time and muscles saved. Find the paint store in town that
services the PROS the Body shops in town, that is were you will get the right stuff and
the right info.
Article Courtesy of www.buicks.net
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