There are many keys to long lasting paint jobs. One of them is spreading body filler over the appropriate surface. This means when you are re filling an area, the previous body filler should be finish in no coarser 80 grit. This does not mean almost in 80 grit; this mean completely in 80 grit.
Sometimes though, there is a couple random deep scratches from roughing the filling in with 40 grit. (See image below). These are the scythe things can come back to haunt you in the paint. Why? Not because filler automatically shrinks in 40 grit, lets be honest, it probably wouldn’t at all.
These scratches almost always have dried dust in them. This is what happens. When you rough in your filler with 40 grit, it isn’t quite dry yet. This ‘not quite dry’ dust gets in these deep scratches and finishes drying, and sort of sticks in the scratch enough to not blow out, but its still loose. This will come back if you don’t catch it!
What to do?
- Take anywhere from 80-150 grit, and sand the scratches all the way down, and feather them out really good. (See above image)
- Then, squeeze filler in the feathered area good.
- Let the filler completely dry.
- Block the area with 80 grit only.
- Use just enough filler to cover the feathered out area.
- I don’t have a picture with the unsanded filler, but you can see the area the back to being smooth and blending right in.