Mixing the body filler is one of the hardest parts of filler work. It depends on many conditions and how fast you work as well, which only makes matters more difficult to interpret.
The only way to get good at it is
to practice. Be prepared to work quickly and also just accept that the filler hardened up before you could use it all, or that it will take a long time to cure if there wasn’t enough hardener and heat.
There are a few tips and tricks that can help when working with polyester based body filler but it is by far better to learn to use it properly in the first place.
Estimating how much body filler you need is going to be hit and miss, and even after decades of using it there can still be incidents. Having to mix more is time-consuming, but mixing too much filler wastes material and time; both equal money.
Prepare your metal the best you can. Get it as straight as you can so that you will use as little body filler as possible. Cave and pave methods of filler work might seem like a fast approach but in the long run thinner is better as far as filler goes. Deep flows of filler can expand faster than the underlying metal and cool faster also. This can cause it to crack and literally fall off the vehicle.
Dolly your panel as smooth as you can get it if you can get to the back side. A stud gun can be used in places that you don’t have access to the backside. However, you straighten the metal make sure it doesn’t oil can and is properly dollied. Oil canned metal pops in and out easily and will make problems for you. It is best to avoid it.
Metal when it bends, especially when it creases, create work hardened edges. The molecules on the outside of the bend stretch apart making it thinner, and moves them to the outer bend, making that part work hardened. These two areas should be dollied to get them straight and even. When an oil can pops back and forth look for the places that are work hardened and the places that are stretched. Try to hammer the hardened places to bring strength to the weaker area that is letting the oil can happen. If not sanding it might be impossible as it moves in every time the block and sandpaper passes it, continually keeping it as a high spot when you feel it.
After you have hammered your work grind everything with 180 if you haven’t already. Feather edge an inch and a half or two inches of paint at least so that it ramps up from bare metal to the paint. Sand off any paint that isn’t sticking or is heavily damaged if it is in the immediate area of the repair if possible.
Mix your body filler very well before you start using it. Open the can up and mix it with a paint paddle. Some guys like to store the new can upside down for a day if they can to help is get homogenized. Try not to introduce a lot of air into the filler. Knead the tube of hardener between your fingers well to get it mixed evenly also. Mixing it well helps create a smoother filler which is easier to work with and gives better results. Using a paint shaker sometimes will create a lot of pinholes in the filler. I’ve used paint shakers before but I don’t like to use them for mixing filler.
Estimate how much filler you will need. There isn’t a hard fast rule and you just have to learn this from experience. Estimating how much hardener you need is also something that you have to learn.
Using a good quality spreader scrape the filler and hardener in a semi circle and fold it over itself. Try not to get a lot of air in it. Once it is mixed pretty evenly use the spreader to put pressure on the filler and wipe it smooth. This helps get air bubbles out which can result in pinholes. Drying too fast also creates pinholes. If your filler gets a lot of pinholes try using less hardener.
Body filler hardens as a reaction between the hardener and the filler. It creates heat. If you mix a lot of hardener into a full can of filler you find that it gets so hot that you can barely hold onto it.
Once the filler has hardened you can start sanding it. If you are going to rough out a badly damaged area and use a cheese grater to knock it down do not wait until it gets completely hard. Once the skim gets a waxy feeling on the filler and you can scratch it with your nail and it feels rubbery begin using the cheese grater. If the teeth get filled with gummy filler wait a minute and check it again. Once it gets hard you can’t knock down a lot of material easily.
Sandpaper will clog if the filler isn’t dry enough. Scratch the surface with your fingernail and if it leaves a white mark it is at least almost ready to be sanded. There is a waxy film on top of some filler and this can clog your sandpaper quickly if it isn’t dried enough. Sometimes it is better to use an already used piece of 80 grit sandpaper to take this off. The longer it dries the harder it gets to sand but at a point, it doesn’t get any harder to sand.
If you sanded your panel or area and it still isn’t straight, add another layer of filler. Sand it until you feel it and it is acceptable. This is another area that experience will just have to work its magic. Knowing when you have enough filler on a dent and when to spread on your topcoat putty.
EverCoat makes good products and the squeeze bottle of putty works great for filling in sand scratches and pinholes as long as they aren’t terrible. Thin coats are better than thick layers. If you need thick layers of filler you should use body filler and not putty.
Sand off the putty, like you did the filler using 80 grit. Then finish with 180 making sure not to take off too much material. The putty layer you will find you have a preference for either leaving a ring of metal between the filler and the old paint, or you will prefer to putty up to the paint edge. Painters often have a preference too. If someone else is doing the priming and painting talk to them and see how they prefer it. Primer can cover up some but it can’t do miracles.
It is now ready for primer and paint prep.