Spent the whole weekend building a Comet out of cardboard

Cardboard builds hold up better than people expect if you seal the card first. I did a Panther hull out of a delivery box a couple of years back and learned the hard way that drybrushing bare brown card just frays the surface and leaves fuzzy edges everywhere. Thin the PVA with water, maybe three parts glue to one part water, brush it over everythign and let it dry overnight. After that the card takes paint like plastic and the scalpel-cut edges stay crisp.

Your individual track links sound like more patience than I have. I cheat with a strip of corrugated card from the same box, run a bead of glue along one flute edge and wrap it around the running gear. The corrugation even reads as link detail from a foot away. Not as nice as your sag over the return rollers though, mine sit flat and a bit stiff. One thing I'd suggest before the drybrush is scoring your panel lines in with a ballpoint pen that's run out of ink. Press hard, use a steel ruler. The lines catch just enough paint to pop once the sealant is on, and it costs nothing.

For the base, a picture frame backer board from a charity shop is sturdy enough and alredy has a nice edge on it. I mounted my Panther on one with some sand from the kid's sandpit and dry plaster dust, been sitting on the shelf two years with no warping.

I'll be honest, I skipped individual links on my Crusader build and went with a scored strip of card instead. Marked the links with a pencil grid, pressed each line with an old biro tip to get a groove, then bent the strip around the running gear. From a metre away nobody can tell the difference, and it saved me a whole weekend I would have lost to tweezers and glue strings. The sag still reads fine if you let the strip hang slightly loose before it dries. One thing on the drybrush plan, go easy on bare card with a hard brush. I chewed up a corner of my hull that way before switching to a soft makeup brush with barely any pigment on it. Paint also warps thin card fast, so if you ever do add colour, do the light coats thing and back any big flat panels with a second layer of card crossed at ninety degrees. My Crusader bonnet went rippled before I learned that. The Comet turret is a good pick by the way, that cast mantlet shape hides a lot of small sins in card. If the Sherman happens, the hull is mostly flat slabs, so it suits your material even better than this one did.

Agree with the sealing advice, made the same mistake on a cardboard Firefly I did last year. Drybrushed straight onto bare card and the turret front went fuzzy like felt, had to sand it back and start over. One thing that helped with tubes: I rolled mine around a wooden dowel and held them with a clothes peg while the glue set, keeps them round instead of going oval overnight. Your Comet turret sounds brave, that sloped front beat me on mine and I ended up laminating four layers just to get the angle.

Not sold on the pen scoring tip. I tried exactly that on a Churchill glacis, pressed hard along a steel ruler like suggested, and all I got was a shiny crushed line where the card surface went waxy. The drybrush wouldn't sit in the groove, it pooled either side and made the whole plate look streaky. Thin card just deforms instead of holding a crease. If I want a panel line these days I scribe with a pin pushed into a cork, barely any pressure, then seal. The line stays open and actually catches paint. Has anyone else found the pen thing only works on thicker greyboard?

Update: I tried sealing the card first and the fuzziness from drybrushing is gone, wich was a relief. But the pen scoring tip idea did not translate well for the tracks; it just made the card surface look crushed. I think I'll just try cutting the links out with a sharp hobby knife to keep the surface clean.