Edit: Really the Estes PSII kits are just fine the way they come as they are fairly light for their size, however adding FG also adds weight, and will change the motors they can be flown on, rockets built with LOC tubing are literally twice as heavy (go figure, the tubes are twice as thick).
Epoxies and the like:
The 30 minute epoxy from your local hobby shop will work or better yet are epoxies by Aeropoxy, West Systems, or my favorite US Composites 635 Medium hardener. The Bondo cloth works but you may have to do some filling after the epoxy cures because of the deep weave, I use it with a layer of 2oz for the top layer, then it covers the weave (bondo cloth is like 7 or 10oz per yard cloth). If you have a local RC aircraft hobby shop they will have some good supplies for trying FG, MY local HobbyTown USA has both .75oz and 2oz in packages (each package has about 9sq.ft.)they also have the Bob Smith laminating resin. Just don't use a polyester resin like HD and Lowes sells in a quart can. The Bob Smith resins are also frequently branded with a particular hobby shops label, the 20min Finish Cure is the BSI brands laminating resin.
https://www.bsi-inc.com/hobby/hobby.html
Your power sander will work OK but a belt or disc sander is better for cured epoxy, whenever I can I try and trim away the excess before the epoxy fully cures and can be removed easily with a Xacto knife, with the BSI epoxy that may be tough to do since the hard cure time is so short. The US Composites, Aeropoxy, and West Systems epoxies have much longer cure times (many at least 12 hours) so you can check a test strip to see when its no longer tacky but still pliable.
My regular suppliers of Epoxy is US Composites, and for FG cloth I frequently use Raka Inc, Fiberglast, or Fiberglass Supply.
FYI the lighter cloths are more difficult to work with when laminating tubing but on flat parts are not too bad, you can apply the epoxy directly to the fin and lay the cloth on it. If you fiberglass the fins then it will be necessary to use epoxy on the fin to tube fillets and joints.
John Coker has a good video on glassing a tube
https://www.jcrocket.com/tube-wrapping.shtml and other really good videos on HPR construction techniques.