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Old 02-12-2006, 03:03 PM
Brian Martin,Freelance adviser
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The most common mistakes made by the newbe working with plastic filler.

The most common mistakes made by the newbe working with plastic filler.

1. They don't give the dent the respect it deserves. What I mean by that is the newbe will usually not look at the dent as big as it is. Not in depth, but in area. The newbe will not see that the impacts effects on the metal don't end right at the impact. There is usually a brow that lurks out away from the direct impact. First off, in the metal working stage, that brow (or many) can be holding down a low spot. This brow may be very subtle. You have to "give the dent the respect" by stripping the paint out past where you think the dent ends. This way you KNOW you are going to be spreading the filler out far enough to cover it. Going out "too far" especially with the final skim coat is not going to hurt a darn thing. That filler out here can be sanded down paper thin and won't hurt a thing.

2. Be sure the filler goes OVER the deep scratches in the metal. And for Gods sake, be sure the surrounding paint is feathered out with 120 or 180 and there are NO coarser scratches left in it. This can cause all kinds of problems as you final block out your filler and find that you "can't stop" because of them. You sand more trying to fix these scratches in the metal or the paint and ruin your blocking you did on the filler.

3. They will put the filler on to thick, and sand it with too fine of paper. Applying the filler really thick (I do this my self on the first coat to KNOW I am filling what I need) is fine, but you BETTER be using some COARSE paper to CUT it flat. If you apply too much and then "polish" it with finer paper you are never going to get it flat.

Try thinner coats applied with more pressure on the speader so press out air bubbles and press the filler into any nooks and crannies.

4. And without a doubt one of the biggest mistakes made is sanding too much filler. I can look at body work most of the time and tell you if it is smooth or not, without touching it, no kidding. If there are shiny spots in the metal in the middle of the filler or on the edges, this is a dead give a way of sanding too much. The feather edge of the filler should look literally transparent. Also, if there are different colors of filler (a little different amount of hardener with each batch changes the color) I can almost guarantee that it will not be smooth. The different applications of filler here and there trying to fix things sand different because of a number of different reasons. This means you could have some filler that is not fully cured on top of some that is. When you sand this new filler and your block runs out onto the surrounding fully cured filler, one is going to sand different than than the other. How can you get them even if one is sanding at a different rate than the other with the same stroke of the sand paper?

If you rough out the work as described in the "Basics" and then apply a THIN skim coat of filler over the whole thing, you are WAY, WAY better off. You sand the whole thing down at once and when you see that transparent edge you STOP. You will need to feel the panel, but when you see that transparent edge, you are VERY close and need to really pay attention because just a few strokes of the paper and you have shiny metal.

This is how I have put into words how you sand the whole area at once bringing down this skim coat.

Picture you have a large mound of dirt in your front yard, from fence to driveway and out to the side walk is this gental mound about ten inches higher in the middle than at the edges. You want to remove it, but without your neighbors knowing. You shave the whole thing at once every night in the dark, paying a little tiny bit more attention to the center, thicker dirt. You do this everynight, a little at a time until the whole thing is gone.

Sounds goofy, but that is something I thought of while sanding filler one day.

Anyway, practice on some test panels and strip the paint out real far so when you feel the filler you are not hitting the feather edge of the paint. Look for that transparent edge.


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Old 02-12-2006, 04:15 PM
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May I add something?

Using too much hardener

Sure, adding a little extra can help cure times in cold weather, but it's a fine line. Add too much, and your filler will set like concrete. This means you'll have to work really hard to sand it back, rather than concentrating on getting it flat.

I've also seen occasions where excessive hardener has eventually worked it's way to through the primer coats, and caused a stain in the base.
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Old 02-12-2006, 05:20 PM
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Way too much hardener may actually cause it not to cure well, I am talking way overboard. Also going overboard on hardener can also mean possible adhesion problems and a lot of pinholes in the filler. Too much can be as bad as not adding enough hardener. The most common mistakes I see beginners working with filler are not applying the filler neatly leaving a big thick area of filler where it will need to be feathered, and not past the actual low area, and sanding too much back off so its not feathered out a little past the repair area. Also working too hard with too fine a grit sandpaper.
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Old 02-12-2006, 09:27 PM
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My 2 cents.

One thing that always gets me is, well finding the actual dent once
it is sanded with the surrounding area !. If you have a shallow small dent I find the following works pretty good. I use 2 visual cues.

Visual cue 1 is I draw a circle or outline around the actual dent with a black

Visual cue 2 is I draw another circle or outline about 2 inch's out from
the previous outline.

Then work the area as needed. but dont go beyond the second visual
cue or your just wasting material.

Doing the above saves me alot of time !

Thanks ><
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Old 02-12-2006, 09:58 PM
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Be careful with them black markers x711, they can come bite you in teh butt later on.

I really think that properly doing body filler is as much of an art as metal shaping is a black magic. Sanding too much was definatly my worst problem when I first started. You really need to learn how to feather it good and accept that to make it true, it will extend way beyone the actual "dent" that you see with your bare eyes. You end up wasting a lot more filler when your learning....I learned that in a hurry.
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Old 02-12-2006, 10:43 PM
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Well the marker is only to show me where im at. What I usually do
is what i call a concave build, I press the filler hard into the dent
then add a bit more so its slightly higher in the primary dent area.
Then feather out from there.

I agree though the first few times you do it there is a tendency to apply
way too much filler, I guess we all have a particular method that suits
our needs.

I think the real trick with filler though is to have as little as possible, i.e
work the metal so it does not need any filling, but thats a whole lot
easier said than done most of the time.. Takes practice !

Thanks ><
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Old 02-14-2006, 09:58 PM
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excellent thred martin sr as usual

Ive made all the mistakes you mentioned when i started back in 1986 .I wouldnt use a marker like someone said but i find cutting it with 80 grit then a skim coat then 120 grit and im there ready for primer.mike
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Old 02-14-2006, 10:21 PM
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The thing I see people have trouble with is changing the direction of the sanding stroke in the same place instead of always a random little different spot. Even when using guide coats they'll concentrate on a fixed "target" flaw and be oblivious to where the sanding block is stopping and changing direction over and over in the same place making low area all the while thinking another area is getting better..
*Guide coats help not only show lows but they also tell a story about how the surface is reacting to our actions and input.
The same idea plays out with the spray gun changing direction on up to the buffer.

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