Got a Black Diamond Mesa–SeamGrip or SilNet?

Isn't the Mesa a 'conventional' double wall tent with factory taped seams? Unless the instructions say otherwise, I would leave it alone. Do a "rain test" before your trip if that will make you feel better — but slapping on sealant and retaping regardless of need may void the warranty, add unnecessary weight, and turn a beautiful tent into an ugly mess…

It is a double wall tent. And it is factory sealed (floor and fly). The instructions say:

The interior seams of the tent floor and the rainfly
are factory seam taped for waterproofness. It is not
recommended or necessary to apply seam sealer
products. If any problems should develop from wear,
seam seal the effected area only, using a product such
as SeamGrip. Follow the manufacturers directions and
apply only to the interior surface of the fabric.

So I guess I don't need to seal the fly or the floor. Should I seal the tent itself?

I will be doing a back yard water test.

One more question…if you do seal, do you seal the seams on the inside like the instructions mention or the outside or both?

The tent fly and tent floor are the two critical waterproofing areas — and in your tent, both are already factory seam sealed (or taped). There is no need to seal the tent itself.

In the unlikely event that you find a serious leak, I would just exchange for another new tent rather than resealing or retaping and thus voiding the warranty.

Over years of use, small mishaps may eventually occur. You can carry a few small tent repair patches and/or a small tube of SeamGrip (not SilNet). When sealing manually, you should seal the underside of the tent fly (or inside of the tent floor). Some people always feel that "more is better" and will seam seal both sides. It's unnecessary, but not harmful either. For a new tent that's factory sealed, I would resist the temptation to "bombproof" the thing further.

When I need to seal one day, which product should I use? SilNet only on SilNylon and SeamGrip on everything else (pack, canopy, etc.)?

Yes, SilNet on silnylon and SeamGrip on everything else.

The reason for SilNet is that nothing else will stick to silnylon. Now, you can use SilNet on other fabrics — the stuff will work for a while — but it will tend to flake off fairly quickly. Thus, SeamGrip for everything else.