Depending on the thickness of the cardstock or cardboard there are at least two other ways to harden them. First: good Ol'e medium CA. soaked into the fiberboard type fins. If thinner cardstock or pressed cardboard I usually apply a soaking coat of min-wax wood hardner.
Once dry either sands like styrene plastic using 240 and 320 grit sandpaper. then prime and paint as you would for any other rocket.
One word of warning: by hardening cardstock it is a bit more subject to cracking and breaking on impact landings. if you intend to fly you models with streamer only it is NOT necessary to harden fiberboard fins.
I'm not sure if the Excaliber you speak of is from the Space pirates series or not, along with the dragon ship they were a pair of "Must Have" models from the golden age of rocketry. This Old brain sometimes confuses the really Good rocket designs between Centuri and Estes. That Said these early really KEWL designs had to be downscaled to fly on Micro Maxx motors. and so they both were
Like you in my early rocket years I LOVED all the Centuri designs. One of my all time favorites is the Laser-X which has a long cardstock interstage shroud my 1971 original and my clone 1: 1.41 Upscale use the same CA Soaked cardstock shroud. In 1990 I found & purchased a 1968 original Centuri kit, and built it anyway. about 10years later I did a 3X Upscale with a BT-101 lower stage. The Upscale is a 3- D12 clustered lower stage with and 18mm (Usually a B6 or C6) upper stage. Photos below show the orignal 1968 model and the 3X -3D Upscale side by side. the Main fins and interstage are actually 3/64th" 3ply aircraft plywood. fins are over a 1/4" foamcore base. in recent years I've switched almost entirely to building and flying ALL my favorite models in Micro Maxx power scale. You guessed it: a 2x downscale Laser-X powered by a single MMX-I motor was added to the fleet.
Centuri had a bunch of Cardstock and-or fibre board models. The X-24 bug BG, The Vulcan, Centuri UFO were all entirely cardstock. Some parts required stiffening while others remained untreated only primer and paint or marker as usual.