nondem wrote:
I've started on my plane!!!!

I have a couple of questions that should be easy for you guys to
answer...

The first one is almost stupid but I'm anal about these things: What
grit of sand paper is ideal for sanding/deburring the cuts I've made in
6061-T6? The saw/blade I used really did a good job to the point that
it almost doesn't need sanding but like I said - I'm anal about this
stuff.

The second question is more technical: I'd planned to use 6061-T6 plate
in .125" thickness for my gussets. I've found a source for plate in the
same sizes but in 5052 w/no "-" number listed.
I looked up the numbers and 5052 appears to have very similar strength
to 6061. Can someone advise me if using 5052 is advisable and if I need
to look for a specific "-" number?
The 5052 is only slightly cheaper but much easier to find...I'm
interested in doing the job right.

http://www.aircraftspruce.com/catalo...s/aluminfo.php

is a very good explanation.

5052 isn't considered heat treatable, that is mostly why there is a "-".
This also makes it a good choice of alloy for welding (it is popular
for fuel tanks). The heat from welding usually results in a heat
treatable alloy (such as 6061, for example) not being tempered anymore
(starts out as -T6 but becomes O after welding, for example).

I would use a file to clean up the edges and burrs. I wouldn't use
sandpaper because bits of the abrasive material may become embedded in
the workpiece and create a corrosion problem later on. For similar
reasons, I've been told not to use a pencil for marking aluminum
(measurements, notes), although a Sharpie pen is acceptable. Then
again, the designer may have taken these issues into account.

The other responses address the question of substituting one for the other.

Nice webpages