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Wouldn't you just start at an opposite end to the tack? So then by the time tmits affected by the heat, there is already some brazed tube to hold everything in place?
Josh-
I'll try and do that where ossible, also will try and tack in multiple places but then it's a balance of more tacks = less places to start that are away from tacks.
@vthejk for me, powdercoating seems to be a common denominator in frames needing repaired. I think there are multiple reasons this could be the case,
1) There's no anticorrosive propertiers to most powdercoat jobs. It is possible to add things to the powder or to prime the metal but most of us are going to coaters more used to doing garden gates and radiators and they aren't offering these things.
2) Powdercoat does a really good job of sealing your frame away from the elements...until it doesn't. Then it does a really good job of holding onto moisture and hiding what that moisture is doing to your frame from you. It could be a flaw in the original coating job (see garden gate powdercoaters who aren't doing amazing quality of work) or some damage inflicted upon the coating as you use the bike but one water gets in there it can travel under the powdercoat and sit around in there.
3) Damage caused at the prep stage. I've spoken to platers and sheet metal workers who have said you can't just blast one side of a sheet because the blasting basically hammers the metal and spreads it out so your sheet/plate gets all warped, you have to blast the other side to get it flat again. I can see correlation between this and frames that have been blasted, coated and have then cracked. Again, most of us will be dealing with coaters that are used to blasting paint and rust off of garden gates not thin walled cycle tubing so while I recognise this is something that doesn't have to be a problem (do it with the right pressure,media and care) I think it commonly is.
M_V
My opinion/thoughts,
I braze tack but you have to watch that you don’t disturb the tack when you start brazing for real coz your working with enough temperature to do so. You won’t disturb a tig tack the same because brazing isn’t (shouldn’t be) hot enough.
Also tig tacks are tiny so are well hidden by the fillet where a braze tack can be noticeable after completing the fillet.