Bondo 101

[firefighter]

I could use a few helpful hints on bondo or body filler.
I've built supercharged, nitrous injected motors, but I suck at body work.

When filling small dents or unused trim holes, what's the best way to finish them? Do you block sand or use an orbital or what?
I feel if I don't sand enough, you can feel the slight lip of the bondo, but if you sand too much you can almost feel the "dip" where you oversanded.
How do you achieve a middle ground where you run your hand over it and it feels like there is no imperfections?

I'm sure this is a way in depth topic, but I'd be happy with just a few pointers.

Thanks in advance.

[80stepsideguy]

1st off DONT EVER fill in holes with filler, eventually it`ll shrink n causes more problems then good.Weld up the holes and then use stuff called USC ALL METAL. It wont shrink, warp or crack.Not cheap but its good stuff.What i like to do with filler is try n get the dent out as much as possible, then skim coat it, i like to use a paintstick and fold the paper over so its flat with the stick.Fill beyond the dent and take the sandpaper and sand it down,take some guidecoat and then sand again.You will see your low spots and then you can reapply to the area that is low. Dont DA the filler cause it wont come out right..Always either block it by hand, use a airboard then sand it by block or paintstick..

thanks
pat

[bake74]

     As 80stepsideguy said, try to use as little bondo as possible, get the area as close as possible with barely a indentation and then fill it, sometimes you need to do it twice to get it perfect.   Never, never use bondo to fill holes, besides shrinking they could fall out/in and ruin your paint job.

[firefighter]

Thanks for the replies guys. I appreciate you taking the time.
I'll be working on the cab this weekend.

[Cabcorner]

 Want to learn about bodywork ,go on U-tube and watch swrnc (diy  bodyschool) ,hprauto ,jamesfreddys,& pisser hack and pack shop. I've learned a lot from these guys .

WARNING some of these guys are eccentric to say the least

[HAULIN IT]

You are correct...This is an in depth topic that is nearly impossible to TELL someone how to do it. You need to SEE & FEEL what is going on. There are some basics that can help though. Definitely don't fill holes with filler as was mentioned.
The biggest problem most people have when starting doing bodywork (unless your doing it at a shop) is that they don't have good/correct tools & quite often don't know what is the best materials for a certain job. The tools part starts with a big enough air compressor to run the tools...this is compounded by the cheaper sanders usually have a high cfm consumption.
While I agree with Pat on the "get a much of the dent out" part, there is a fine line here & a newbie can get in trouble VERY easy. I definitely don't agree on the "use as little of filler as possible" statement, that mentality leads to problems also. You need to be able to identify the Three possable levels of  a panel. (1) Low spot (2) the correct level (3) a high spot. This sounds easy, but to many it's not!

Here's the Two simple "rules" I tell everyone that will listen. Rule number One...You can't fill a high spot! Rule number Two..."Bondo is your friend!
To explain Rule #1, it means just that! You, I, He...nobody can FILL a high spot. Only low spots can be filled. Guys everyday put glop over a high spot & fake it in, paint it & call it fixed. Reality is...sometimes this is what is needed due to pressure, stamping, understructures, ect. but what really happened was: a larger area of filler was used to gradually change the panel to make it appear smooth. The best plan is to make the high either level or slightly low...not work over/around it.
Rule #2 takes a bit of explaining also. What I'm getting at is that unless a person is highly skilled at metal straightening, body filler is needed to remove flaws in the surface. That being said, a person can't be afraid to put it on. Bake mentioned "sometimes you need to do it twice to get it perfect"...I wish it was that easy! It needs done until it right, there is no "number of times". It's just like any other skill, trade, craft...a guy that is experienced can do it in WAY less time & likely end up with a better job than someone just starting out.
The problem with the "use as little as possable" is this leads to what I call "cork in a bottle". The spot of filler is too small to get up out of the dent so it leaves a rather distinct line around the edge.
The opposite is the "pancake, hotdog, football" or whatever else it looks like to me on a certain day. This is where the filler is mounded up & not sanded down level for some reason...often resorts back to Rule #1 but could also be because of someone got tired of sanding (back to the tools/compressor) or used too fine of grit paper & "polished" the spot instead of knocking it down.

Now to get to the direct answer to the original questions. You'll want to block at least toward the end of each coat to make sure your where you need to be. If were talking small, shallow areas (maybe a foot in diameter or less) use only a hand block until you understand what your looking for. The guidecoat is a great way to "read" what is happening. Black lacquer spray can works well. The Black gets removed very quickly off of high spots, remains in low spots & takes a bit of sanding to remove from the level surface. Wiping/spraying a panel/area of repair with a wax & grease remover can "wet up" the panel so you can look into the shine can also help.
Here's the One thing I can say over the internet that can help in a big way. When looking at your spot of filler "Can you take a pencil & draw around the edge?" If so, your not done! What I'm getting at is: When feathered in to the surrounding surface, the spot needs to be thin (somewhat see through), scratchy, ect. & you can't draw a distinct line around it. A low spot of filler - aka: "cork in a bottle" either comes from too much sanding or not enough filler in the dip & can be drawn around...it has a definite, distinct line which is why it can be seen in the finished product. The same is true for the opposite...A high spot sanded down around the edges trying to make it blend in has a distinct line since the filler is coming into the surface at a different angle & shows up for the same reason.
Long winded I know...Hope it helps in some way! Any other question...Just ask! Lorne     
 

[firefighter]

Lorne,
Your reply was not long winded at all. I read through it multiple times.
I thank you so much for taking the time to write it all out for me.
I especially like the idea of the guide coat. So far I've been going "by feel" to know how much to sand (using the WRONG sander), and then I felt like I had oversanded and had to do it all over again. Some spots I had filled and sanded 3 times already and they still feel like a low spot when I'm done.
I am going to be doing some spot repairs this afternoon with a little more information to go with now.
Thank you all so much.