The captions show how to mix the filler, but when applying it, try to keep it smooth with long pulls of the spreader if you are covering a large area, such as the top of a front fender with many subtle waves in the sheetmetal. You’re aiming for a thin spread of filler, so ensure any large dents are repaired beforehand. The smoother you spread it, the less work you’ll have sanding most of it off. And sand most of it off you will. I like to use 40-grit paper on a DA sander to “knock down” the filler to a level where I can take over with 80 grit on a block, and complete the job by hand. This takes practice, especially on curved panels. Longbed air sanders are available, but I’m used to using a DA. If you find you need to add filler in some areas after sanding, re-apply a spread over the whole area, not just the low section, as it’ll make it way easier to block it flat than trying to block an area of fresh filler in the middle of already sanded filler, as you’ll always end up sanding the new and the old.

Remember practice makes perfect, and this is a skill like any other, despite body filler’s reputation as a cure-all for dents and rust. Always make sure it is used over good, clean solid metal (or ’glass) and get the sheetmetal as straight as possible before applying the filler, which should never be more than 1/8-inch thick.