Bondo Basics – Apply and Sand
Bondo Basics – Apply and Sand (part 2)
This is a follow up to: Bondo Basics – The Art Of Body Filler
When applying and sanding bondo your gonna want to be fully assured you know what you’re doing. This will save you time and materials in the long run. Don’t bother starting with the bondo until you have made sure your damage is pulled, patched, ground down, blown off, wiped clean and ready.
To Begin you have to make sure your area you are applying the body filler to is completely roughed up. Grind the area thoroughly with 36 grit sand paper or at a minimum of 80 grit. Take your air gun if you have one and blow off all the dust and grit from grinding. There are steps people like to avoid, I’m guilty too, but you learn some shortcuts pay and some don’t. Here’s one commonly overlooked. Wipe it down. Take the wax and grease remover and treat it as though you were painting it. The bondo has to stick completely or it will chip and flake, in other words it won’t feather for you and it will tick you off and waste time and materials. If your damage is repaired right you shouldn’t have to apply more than two or three coats to have a finished repair.
Alright, to the tools needed. Generally you want a 5 – 9 inch sanding block. The sandpaper you need will be the common 36, 80, 120 works well sometimes, 180 then 320 for feathering before primers etc. A DA Sander is used by some, but the same sandpaper applies, choose the same grits.
The methods: Depending on the size of the damaged area, mix only enough to fill the damage completely. Be sure to apply enough – this takes a little to get used to. One important note is that you will not spread the bondo perfectly, it shrinks in an you will end up with a low spot. Apply enough body filler so it’s built up a 1/8 or so. Not more than that, you will be sanding your butt off, this gets old 
Take your 36 grit or 80 grit, depending on the amount of area you will be sanding. If you think you can lightly sand and feather in one or two coats, then stay away from the 36 grit unless you know when to stop. It puts some pretty deep scratches in the bondo and the area around the bondo where you will be sanding over. Use 80 grit to avoid excessive scratches. For most larger dents you will want 36 grit and your larger or largest file board.
With the panel prepared and the bondo spread, let it harden. If you have a mess you can carve on it with a knife to get some of the stragglers you slopped on it off. If you wipe on body filler when it is not set up it will surely stick and you’ll have to sand them off. Let it harden and pick it off later.
For large dents that you know will need at least 3 coats of body filler it’s best to have a rasp or body file hand plane. It works somewhat like a cheese grater. Let the bondo set up real good, feel the bondo and it should be a little warm but not cold….if it’s cold it will be too hard for the rasp if it hardens. Take the plane and begin filing in one direction. This is very rough but effective way to cut your panel. This takes a little getting used but it will save you quite a bit of time, sandpaper and your lungs from tons of dust. Use it on the first coat only, the rough coat. You will be turning right around and spreading another coat of bondo over it, after you blow it off real well of course . This is used to help get the filler straight. The low spots will not be filed. Note: To help Identify the low spots… spray the filler with a cheap flat black paint, the cheapest fastest drying kind you can find. DO NOT USE ENAMEL SPRAY PAINT, you don’t have all day to let it dry. Find a cheap lacquer paint, Walmart brand works good at .94 cents a can. Simply dust it, don’t put a full coat of paint on, just enough to add contrast to the body filler to identify where you have sanded. This will greatly assist you identifying what and where you are sanding. High spots are gonna sand first of course. Keep attacking them as you sand until all the guide coat paint around them is off. If you have sanded the filler and panel is free of the spray paint you applied, chances are you are real close to a finished repair. But there are other things to look for and consider.
Take your inline sander or manual long board of choice or hand sander and begin sanding in a criss-cross (X) pattern. This is extremely important if you do not want waves. Sand at a 45 degree angle in all directions to each other. This helps cut the filler evenly. If you sand in a straight line…. you get a straight line. This is not rocket science and there are no tricks. You have to watch what you are doing. Don’t go nuts sanding, do it methodically. Sand And Feel, Sand And Feel. Pay attention to body lines. They will have to be included in your sand and fill process. For straight lines use tape and sand up to it, reverse and sand down to it. Use tape when applying bondo to get a straight line. Apply the body filler over the edge of the tape and remove the tape before it hardens. If the filler hardens you’ll be sanding the tape too, so pay attention.
Apply bondo as many time as it takes to fill the low spots. If you have high spots you’ll know it. If the metal is showing and the bondo isn’t sanded… you have high spots. These will not go away. There are two ways to deal with this, only one is the right way. You can fill the whole panel up with body filler and sand and sand, or you can take your pick hammer and carefully tap them down. Tap them down!!! Don’t be afraid to use the hammer, you will need it until you are finished and satisfied. Don’t cheat, if it’s high… tap it down lightly…tap, tap, tap. All the bare metal showing is high so tap the bare metal and beyond an inch or so. Stop sanding if you see this, they won’t go away and you’ll be getting further from straight. You’ll have to tap them all down and apply more body filler.
When you know your getting it close switch to the light grits. Use 80 grit paper and feather all the edges of the bondo and the paint together. It may feel a little rough and that is normal. You are filling the dent, the primer you use next will take care of the small scratches.
Another note: When mixing the bondo try to avoid flipping it over like taffy. Roll it over and spread mix it. You don’t want to let air get trapped in there. You will end up with pinholes eventually. Pinholes are just that. Small little pits in the bondo. These can be filled later with a glazing putty, but try to avoid excess air when mixing.
After the 80 grit sandpaper begin to taper down on the sand paper. Use 180 to feather any scratches in the paint near the filler. Finish with 320 to assure you won’t have sand scratch swelling later on down the road. The smoother you get the panel from here on out the nicer the repair will be as a finished project.
Wipe it down, look for pinholes, fill them with glazing putty if you have them. If not get that primer ready and spray on 3 good coats. Dust your primer with the black spray can and begin sanding again. I’ll write about the primers next.
Good Luck with your project. You’ll learn from experience, you can’t screw it up if you know these basics to bondo. It’s easier to do than explain
For further reading on Body Fillers go here: Bondo Basics – The Art Of Body Filler (part 1)
