i have recently decided to paint my 79 f-150. i have some rust on the front fenders and in the groove that runs all the way down the truck. When i put bondo on it i can never seem to get it to come out even any techniques or tools that would be more handy.
im using bondo brand filler. its the curve that runs all the way down the truck i cant sand it evenly with a sanding block so what else should i use

 

Hey Devin - Welcome to FTE.

A few questions for you.

1. What are you doing to eliminate the rust and to prevent it from returning?

2. Are you putting the filler right on top of the rust, on bare metal or on a primer surface?

3. What type of body filler are you using?

4. What grade of sandpaper are you using?

 

This is a very tough area to get right. I spent a great deal of time on mine and it did end up very good.

I started by working the metal as close as I could get it, then treating the metal and epoxy priming it. I use a 4' straight edge to give me a reference.

I use Rage lightweight filler and put heavy scratches in the primer to get better tooth.

I applied the filler then used a straight line sander from Harbor Freight ($40) and it took MANY tried to get it close. Towards the end I used evercoat 2 part putty glaze and 180 grit paper to work out the small stuff.

I don't know how many times I had to fill and sand, but I can tell you it was A LOT! I spent at least 2 weeks on that one side alone working several hours a day.

One other note is that the bed has a curve to it, it bends out as it comes to the wheel then goes back in a bit as it gets to the end. Take the 4' straight edge (Home Depot $10) and look at another bed for reference.

Also, I used the 4' straight edge as a guide to apply the filler (ruined the straight edge, but saved time over the small paddles)

Using a guide coat really helps to get it straight too.

 

Here's my process for using body filler:
1) Apply filler
2) Let filler set and remove scum coat with cheese file
3) As soon as the area has been roughed out with the cheese file follow up right away with 40 grit. Note: you never sand filler with 40 grit when it's hard, too much work. If you get the filler at the right time, it will sand like butter. This makes sanding the shaped areas easy.
4) At this point the filler is starting to get harder. Hit it with 80 grit. If the paper does not clog then keep sanding. Note: always have an object such as a stir stick handy to rap the sanding board. If you are hitting the filler at the right time the moist filler will fall off the sanding board, sand, rap, sand rap.
5) at this stage the filler should still be a bit moist. I like to let it set up hard (about 10-15 min) then use the 100 grit.

A dent the size of your fist should not take any longer than 30 minutes to sand.

The only time I dont follow the above is when I'm working on dents the size of a dime. I prefer to let the filler set then follow up with 80 then 100.
The final grit you use depends on the primer surfacer. Some folks like to use 150 rather than 100 grit. Some solvents in certain paints are very strong and will cause swelling of the substrates. This results in sand scratches showing through to the top coat.