Pykirk wrote:

I tried the trex-suggested method of screwing below the surface and tapping the
trex down over the screw head with a hammer. I really think it looks bad.

I agree. It looks like exactly what it is - a munged screw hole.

I'm looking for alternative methods for securing Trex decking.

I put my Trex deck down with 15ga, 2-1/2" finish nails and Liquid Nails.
The tiny nail holes are essentially invisible, and the nails have
plenty of holding power (the Liquid Nails was just for insurance). I
used 3 nails into each joist. My deck is ~440 sq ft, has been up for 5
years, and is absolutely rock solid - not one piece has worked loose
anywhere. Many have commented on how nice it looks to not have visible
fasteners. I'd highly recommend this method.

My building inspector had no problem with this method, but if you're
concerned you could check with yours before using it.

I'm also applying a 1x8 Trex facing material and I am
thinking about making some Trex plugs and drilling out for the screw heads. Is
this a good idea?

Again, I'd use some finish nails and be done with it.

An issue to be aware of is that Trex moves a LOT with temperature.
Where I've done miters on railing tops they have opened up somewhat; my
few glue ups (I laminated & routed pieces to make hand grips for stairs)
have started to separate due to the expansion and contraction. I'd
expect you might have similar issues trying to glue in plugs and keep
them flush & snug.

Will I be able to sand these down when done?

Yes, use a fairly coarse belt on a belt sander. But see above.

I see that
sanding causes immediate discoloration, but will the color go back to "normal"
in a few weeks?

Yes.

-Brett