Estes EAC VIPER #0820, BT-55 Upscale, Part 11, White Undercoats

Here's the upper launch lug after the first white undercoat.

The lug is a bit fuzzy after the filler was sanded down.

The fuzzies were removed with some light sanding with 800 grit. They won't be noticed after the next white spray coat.

The lug is actually straight, there is some camera distortion.

After the first white undercoat, 

The entire rocket gets a light sanding with 800 grit.

Why not 400 grit? 

Some brands of 400 grit sandpaper will show scratches under the paint on the finished model. 

The picture shows the before and after. 

This between coat sanding pass is to smooth out glue boogers and rough spots that show up after paint is applied. Notice the small glue blob at the middle of the root edge.

Over most of the model you end up just dulling the gloss. Don't sand through to the tube or bare balsa.

Take a look at the open ends on the top and bottom launch lugs.

TIP: If you see fuzzies, clear them by rotating a sharpened dowel or pencil point.

You will be compressing the fuzz against the inside wall of the lug. It can reduce some binding against the launch rod during the initial boost.