I'm in San Antonio, about 10 miles from the Sun.  It is hot here, sometimes cold.  

I need to repair the outlet plenum made of duct board, it is coming apart.  I don't know anything about mastic, fiberglass tape, duct board etc.  

Should I just re-mastic and re-tape the existing duct board or take it all down and get new ductboard?  Less hastle is better.  The top piece of duct board has fallen down into the plenum and so I'll have to fish it out and will probably replace it.

Is stapling the duct board together necessairy on 90 degree corners?   On any other corners?

Is there a good way to ensure that the mastic and or tape sticks permanently?  ( what the hell is mastic anyway? )  

Also need to replace the Flexible Ducting that has been killed by UV.   It had gray plastic on the outside and that died...of course...from the turbo ventilators light leakage.

There is black plastic UV resistant insulated Flex Duct and also Metalic insulated flex duct.   I'm figuring lined on the inside is better from a dust/mold standpoint.   What should I use?

Any other considerations?

Any and all advice is welcome.  

Thanks.

First, if you can afford one, get one made from sheet metal and islulated on the inside.  It will cost more, but will last forever.

If you can't go this route, buy a knife made for duct board.  It has 2 blades, and will cut a 90 degree groove, so you can fold the duct board to make square corners.  Use the foil tape.  

Use the foil faced flexible duct.  It seems to hold up longer.  They all have a plastic tube inside.  When you replace the flexible duct, make sure you tape the inside tube to the duct, and then pull the insulated portion down and tape it also.

Thanks for the reply.   Good info.