If you don't want your primer to shrink, do the work in Puerto Rico, in the
summertime, over a six month time span.

Shrinkage occurs for a variety of reasons:
1.  Primer is put on too thick. The first applications of primer need to be
worked down into the weave minimizing the places for primer to shrink
into. Put it on with a foam paint roller and a squeegee. Thin the first
couple of coats and work them in (especially you ES and 320 guys) prior to
fire-hosing on 100 pounds of dead weight. Don't expect primer to cover less
than perfect micro work. If you plan to fix bad micro work by putting on 26
full wet coats of primer, you didn't do your micro work right. You also need
to go on a diet.
2.  The body work is done too fast. All brands/types of primer need to be
allowed to dry fully BEFORE SANDING. If you sand primer that's not dry
and it shrinks back into the weave, whatareyagonnasee? Follow the
manufacturers directions regarding flash time, film build and recoat time. I
like to wait a week before sanding, if possible.
3.  Heat. Heat will cure the primer faster. In Tennessee, in the summertime,
I'll roll a primed airplane out into the sun for a couple of days prior to
sanding. In those two days, you can actually see the primer shrink back. The
WLS (epoxy) won't shrink as much as the polyester or urethane products,
but it all shrinks. I can see weave in things we have painted in the
wintertime more readily than projects done in June.
4. Polyester skim coats. Some people like to skim coat all their micro work
or bare weave areas with spot putty. My experience is, polyester is the most
shrinkage prone product out there, followed by urethane, then epoxy. I
didn't even mention lacquer, as it was out of date 30 years ago. You can sit
and watch it shrink, if it doesen't fall off the plane. If you must skim
coat, use a catalyzed spot putty product.

The best defense against shrinkage is to shim coat the entire surface with a
thin layer of micro. Lots of work, but better and lighter than using bondo
or a 55 gallon drum of filler-primer.

When applying micro, always paint a light coat of resin first, then apply
the micro as dry as possible
Fluorescent lights are always going to show all the imperfections. Even
mine. If you can make it look great under fl. lights, I'd like to offer you
a job. You will still be the guy that wet sands the belly.

Brad Simmons
EAA/Sportair Lancair Instructor
Airframes Inc.
Milan, TN