When working with body filler, what ever tools you use, the trick is to not add the last “skim coat ” till you KNOW that it is all you need. Don’t try to block out that first coat, just use it as a base for the LAST skim coat. I was taught this procedure after doing bodywork for a number of years and it really works well:
Just apply a nice coat of filler (what ever brand, whatever style, (we will put that aside right now). Cut that coat NOT to make it perfect, but to get the basic shape and filling you need as a base for the skim coat. You can cut it with 36, 40 or 80 depending on how big the area you are working is. In other words, if you can cut it fast with only 80 then do it. But I would say that this would be limited to an application that is no larger than about 8 inches.
If you happen to have a few high spots, see if you can tap them down. If you have a few low spots add a bit more filler to ONLY those spots.
Re-cut these last low spots you have just filled with the same grit you have been using (most likely 36).
If you now have a surface that ONE skim coat will fill, then apply it. If you don’t, work with it a bit more, but NEVER add a little here or there and think you will finish it without a skim coat.
If you have a surface that is very close with only a few VERY MINOR low spots like poor feathering onto the metal, poor transitions from one application of filler to another, or from the metal that is “poking” up here and there you can do the LAST skim coat.
This skim coat is very important, you want it to extend over the COMPLETE area, this is well past the damage you have been working. Maybe as much as 3 inches past the plastic that you have applied to “rough” it out.
This skim coat can be regular filler or a polyester glaze like “Icing” or “Polyester glazing putty” , that is your choice, I use both depending on the size of the area being worked. Do not use anything that doesn’t mix with a hardener. NO , “Spot putty ” in a tube, only polyester putties or fillers. If it uses a hardener, it cures to a hard film. The “spot putties” stay soft and can become even softer when the solvent from the primer coats it.
You now run a block, long board, or hog even over this skim coat with a little bit coarser paper than you plan on finishing with to cut off the resin that has surfaced in the filler. I usually just use the 36 or 40 or whatever I have been on the “rough” work. BUT take CAUTION not to cut much off, you want to JUST take the very top, don’t really sand AT ALL .
Now finish sanding with your longboard or block or hog or whatever using the finer paper like 80 on a large area or 120 on that small 8″ sized area. Block it out to perfection with a nice feather edge to the surrounding metal.
I can’t stress enough, the trick is to know when just ONE LAST skim coat will do the job. And apply it COMPLETELY over the surface. If you only have one little low spot in the middle, don’t just do it, skim the ENTIRE thing. You must have one LAST skim coat over the ENTIRE thing every time. If you get in the habit of this you will do it over and over on every dent you repair and find that you can do just about any dent with just two applications.
As you sand the filler let the board or block you are using run over the surrounding metal. If you only work on the filler you will sand it too low. You need to keep it as high as the surrounding metal, so use the metal as sort of a straight edge that you run the block or board off of.