Can you guys tell me what is a good sanding sequence in terms of Grit.
this was sarted with 40 then 120.
?????????? I need input.

 

Roughly...
When you are removing material use the coarsest
abrasive available...

This means grinders, flap discs, wire wheels,
abrasive blasters and acids

Once everything is gone that I wish to remove
I start filling the larger voids with the fillers of my choice..
Next I add a guide coat of rather thick primer...

I sand with 60 to 100 grit to get some idea of where the
voids are and then fill them

The car is still "mostly" bare metal with large "spots"
(around a foot in diameter or so) where the voids are comming "up"

Now comes the "primer coat" and block sanding (wet ) from 150 to 320 Its "scratchy" but the voids are filling.

Now comes a guide coat in the "color" of choice...

Block sanding, wet through 600 grit...

You should now be close to "paint ready"

 

I'm doing the whole car and that pic is from today which is when I started.

 

Keep in mind that using 36-40 to strip a panel might also ruin any galvanizing (zinc anti-rust coating) that was put there by the car maker. I sort of do the opposite; I spray primer first, then I spray a "mist" of a guide coat. I usually use black primer for the guide coat instead of black paint. Then I sand the guide coat just like normal primer, with 150 or 220 grit.

I sort of go like this;

40-60, stripping
100, final bondo sand
150, first primer sand, guide coat sand
220, "middle" coats of primer, any other guide coats I feel like shooting
320, "final" coat of primer, wet sanded

Then, paint! Some guys go up to 400 grit before paint; I think the 400 doesn't give enough "tooth" for the paint, so I stop at 320.

 

I think Tom is also "right" ...
With respect to tooth...

Adhesion is somewhat tricky and to a large extent
depends on the materials you are asking to
adhere to one another.

Tom is advocating an increase in surface area
as an aid to adhesion. Particularly important
when attempting to use a "sealer" to seperate
incompatable finishes.

In the particular case where the surfacs are relatively
"fresh" you may get away with less "tooth"
particularly when working within the same family
of products (ditzler, dupont et al)

I find that going higher on the primer/sealer
allows the color and clear to lay up more evenly
and so I am more comfortable with relatively
thinner coats
( I am also less likely to try to compensate with the gun
which leads to sags )