I would like to know the stance of the Federal Highway Administration on
temporarily covering signs that aren't in use. More specifically, what does
the FHWA suggest crews use? The reason I ask is because plastic bags blow off
(or the tape adheres to the sign) and plywood is heavy which means one of the
big guys has got to put it on the sign.

I'm not speeking for the FHWA, but I think they'll say that there are two main
concerns with sign covering:

1) The sign face is completely covered
2) The covering does not pose a hazard to run-off-road vehicles

The first is a little more difficult than it might seem. Thin plastic or open
weave burlap can allow enough light through that the signs will actually
retroreflect through the covering, and be visble at night.

I've seen plans that require flexible coverings to be weighted down. Such
weights could conceivable go through a windshield in a crash. The same could be
said for plywood panels. To pass NCHRP 350 crash testing, any cover will have
to be either soft, or securely attached to the sign.

Regarding plywood covers and crashworthiness take a look at question 3.3 on the
linked page which would seem to indicate that plywood covers are not
prohibited.

sorry here's link safety.fhwa.dot.gov/roadway_dept/road_hardware/qanda.htm

JJM covered the basics: completely covered and crash-worthy. We have used old
signs, same size as the one to be covered, but I doubt they are crash-worthy.
We have started using either corrugated plastic sign blanks or 1/8 inch
hardboard or "luaun" (not sure of the spelling there). To attach the covers, we
use lightweight metal hangers or hooks on the top, then use lighweight spring
latches to hold the bottom against the covered sign. By placing a large picture
hanger on the center of the cover about six inches from the top, you can then
use a wooden pole or dowel to lift the cover in place. For a cover that has to
be up in the morning, down at night, we attach the cover to a one inch dowel,
hang the sign with the hooks, then secure the dowel holding the cover to the
permanent sign post with cable ties or rope. Both of these methods avoid having
to use heavy materials or set up a ladder...both of which could be a safety
concern.

Lauan plywood? Named for the lauan trees of the Philippines that were the
original source. Also spelled "luan".

Just turn the signs through 90 degrees so drivers can't see them? Or lay them
down on a plastic bag so that the face doesn't get scratched? Or roll/fold them
up? Isn't there some sort of temporary grey material that you spray onto a
permanent sign face that can be rubbed off when the works are complete?

There are inexpensive commercial products that are quick, easy & durable to
install/remove as well as completely opaque even under headlights. (There are
also similar looking products that don't meet all these criteria.) The most
popular methods locally are turning the sign 90 degrees, fine if it's on
temporary (movable) or portable supports and can be secured to stay in position
while crash-worthy, or laying the signs down and away from traffic.

Here is the internet address to approved sign covers for Oregon DOT - if you go
to the QPL Home site, you can find the specs for them.
highway.odot.state.or.us/cf/QPLSearch/returnInitialSearch.cfm

Shouldn't contractors be REQUIRED to cover unused construction signs?

We had 45 MPH speed limit signs 3 months after construction was completed on
the parkway. Most people were going 75 MPH on Sunday afternoons.