Originally Posted by
bulgie
Yes, wherever you are on the ham-fisted to godlike skill spectrum, all else equal, steel with higher surface hardness should be safer against thinning the wall locally while sanding the fillets. OTOH those tubes tend to start out thinner too though, so even a little thinning is a higher percentage of the total thickness lost to make the fillets look pretty.
I think we can all agree that no matter how good you are, smoothing the fillets to make the edges disappear will inevitably thin the steel, at least a little. No one can keep the abrasive or other tools 100% away from the steel, unless the edges are left visible. As I guy who's cleaned up a lot of fillets, I'm not saying Don't Do It, just consider keeping it to a minimum. Maybe let your painter do some of it, with sanding the primer or even a little painter's putty. No shame in that if it makes for a better, longer-lasting frame.
I think 631 is one of the only steels still being made in 1/7/1 but I don't think they're all stocked here and the 25.4 top tube is limited to 8/5/8.
Even though big fillets are mostly for aesthetic reasons, I think most customers draw the line when it switches over from functional craftsmanship with an artistic flair to pure and unnecessary aesthetics. I do think bondo fillets would probably result in a stronger frame due to heat issues. Yet another reason for 631/853, as I don't think you can realistically or safely hit the hardening window for 4130 with the way a big fillet is done, even if you do fully wet out the filler and don't touch it. But no customer is ever going to be happy with bondo fillets any more than a customer would be happy if you glued some fake plastic lugs on before painting.
It does remind me of a stripped Jo Routens frame I saw pictures of though. Jo Routens tended to use very small fillets except at the headtube. Anyways, this one had some kind of silver-gray filler, the owner said it was gas welded. I couldn't help but wonder if it filled in with some low temperature solder like they used to do for auto dent repair. Much lower temperature and it's actually shaped with wooden tools instead of just being allowed to flow.