For 99.9999% of TF2 weapons, paint doesn’t enter the picture. You paint cosmetics, not weapons, because paint would absolutely destroy the ability to recognize weapons on sight, right?
But what the hell, let’s just add it in by this point. There’s gotta be a way to make Decorated weapons paintable. After all, that remaining .0001% of weapons consists of an elite few weapons that have in one way or another been painted.
The most famous of course are the painted Cow Manglers. Two Team Spirit and one Balaclavas are Forever, they all came into existence in 2011 during an extremely short period of time after the Cow Mangler was first released. Valve allowed them to be painted for maybe 45 minutes, at the time an expensive decision for anyone hoping to buy the new team-colored paints. These are the most famous because the paint works; you can actually see the Balaclava one fires darker rockets that are freaking hard to see.
These things are valuable. They broke the economy when they first came out, skyrocketing instantly to the highest-tier items in the entirety of TF2, and they’ve never really come back down. Nobody who owns one is selling, so good luck getting one. But your chances are still better than if you were hoping to own either the painted Vita-Saw or Ubersaw.
I have tried and failed to locate any evidence regarding where the Vita-Saw has finally ended up, but several sources note that the painted Ubersaw was deleted. The Vita-Saw was painted orange and appears in an old TF2 Outpost trade. Neither of these weapons had any in-game appearance adjustments, unlike the Cow Manglers, and nobody’s 100% sure how they got the opportunity to exist in the first place.
Last but not least, there’s at least one weapon that deserves mention despite never having been paintable. The Lollichop was the first weapon that came out with paintable models, meaning if Valve wanted to they could have made it paintable by just flagging the option on. But for some reason they kept that feature deactivated. I personally suspect that they knew they were pushing the envelope with the entire concept but hoped that Pyrovision would excuse it, but ultimately decided it wasn’t worth it. Perhaps at the time they thought the artstyle still meant something.
But regardless, that cat’s out of the bag, through the woods, out to pasture and already bred an entire herd of technicolor bastard kittens. Paintable weapons could only add to the insanely-high levels of customization Valve has piled onto the stock weapons. And this, like Decorated weapons, should really stay stock. But seriously let’s do it. And while we’re at it let’s steal stickers from CS:GO and complete the package. I won’t be satisfied until every single polygon and pixel of my stock weapons are different from the ones on a normal-quality variant.