bondo / body filler advice

i am filling in a dent on my gas tank currently and i just cant seem to get it to work.
i let the bondo sit until it dries all the way

then i use a coarse 3m sanding sponge

followed by a light grit sandpaper before applying primer

it always feels smooth before the primer but when the primer dries i can tell where i put the bondo

any suggestions?

 

What you're seeing isn't because it isn't smooth but because you have radical acceleration of curvature (can you tell I actually did research on this for a car company?).

When you look at a shiny surface what you see is the reflection of the environment. If you are mathematically inclined you might understand that what you actually "see" is the tangent. In other words it's like an infinite number of little flat mirrors.

Because you're looking at the reflections what you actually see is the acceleration of the curvature of the tank, which in your case isn't smooth. When the surface is non-reflective you actually see the surface which is smooth.

People that design cars put a lot of effort into this.

Even though you can't really see that curvature until you prime, you can feel it. Problem is you're feeling for the wrong thing.

Make sure your hands are dry and gently run your fingertips across THE WHOLE REPAIR. It's actually easier if there is some dust on there (the dust from sanding is perfect). It'll either feel like a bump or a flat spot.

My money is on flat because you're working with a convex surface. You probably need to fill it deeper. Then, as you sand take long strokes running from metal, across the filler, onto the metal. More like you're trying to polish the whole tank than sand the filler.

 

Try filling a bigger area. If the dent you can see is the size of a quarter, than your actual fill area should be 2 - 3 times that area. The dent is a lot bigger than you can feel or see. Scuff the area with at least 36 grit. fill your area well beyond the visible dent. Like said before, sand from metal, across filler, to metal holding your block as flat as possible, and parallel with body lines in the tank. You can start this with 80grit moving up to 250grit. Use a soft glove or paper towel and rub different directions accross the dent, don't look at it, look off to side or up in the air and you should feel it. After smooth, prime with a sandable primer, block primer with 600grit and paint.

 

Normaly start with coarse production paper to get it close to what you want, i would normaly finish using 250grit. Then come the hard part, switch to wet & dry because you will never get a realy good finish just useing dry sand paper. I generally start at about 250 grit and finish with 1200 to 1600 grit. Once you get close you need body putty, its a very fine ready mixed filler, it looks just like thick primer. The body putty will fill any small scratches and let you finish the edges to as near perfection as you can get(depending on operator skill). I would also reccomend using a sanding block because just using you hands you will never get even pressure where you are sanding

 

sometimes the primer just dries different from bondo to metal
try another coat of primer then wet sand and see how it looks

 

I've have good luck using a "Filler Primer". It's thinker so you can sand it and fills in the pin holes. Oh and I use a spot putty to fill in the pin holes in the bondo. Spot putty is thinner and dries in minutes.

 

You can't get a "Scratch free" surface to paint without doing all the Prep.
All the Prep means sanding the surface smooth and matching the contour by sight-lines and touch.

Priming the area to be painted after the sanding is done.
Let the Primer dry or bake it on with a heat lamp.
Wet sand the Primer with 600 or 800 Wet-O-Dry finishing Paper.

When you get done with the Wet Sanding there shouldn't be a swirl to be seen.
If there is ... treat the area with a "Skim Coat" of Finishing Putty.

The Finishing Putty is similar to Bondo but very thin material that fills swirls and pinholes.
Wet Sand the Finishing Putty OFF
What is left behind has filled the swirls and pinholes.

Happy Paint job.