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Finish is rattlecan, possibly plastikote, done some years ago - it now looks a little tired where bungee hooks have worn through to the metal, and I keep meaning to give it another coat when the weather improves. Or not, and call it patina.
Propane can be marginal at the kind of section you'd want for a rack, and the flame from my GoSystem AP2010H is probably quite diffuse compared with MAPP specs. Firebricks can make all the difference, as can care when clamping the work - too close and your clamp becomes a heat sink. The thing about a home made fire brick is that it uses less gas and is cheaper than buying a bigger torch.
A lot of mild steel comes oiled or lacquered to prevent rust in storage - this needs removing a good 50mm either side of each joint, if only to avoid setting off smoke alarms. I usually remove the lot before doing anything else, using an angle grinder with a wire brush, and the same angle grinder+wire brush to clean up after brazing / before painting.
Flux should be a smooth thick paste, so that a generous amount can be applied close to the joint - I try to stay within a few mm to reduce the area where solder or braze might pool on the outside rather than flowing into the joint. Painting flux on to the actual joint surface before assembly often works better than clamping and painting flux on afterwards.
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MrE
Looks very tidy! Is that powdercoated? I was going to weatherproof mine in as cheap a way as possible, which might be Hammerite. Even cheaper would be not using it in the wet.
Re. the fire bricks - did you find that made an appreciable difference? Some internet sources recommend them but it's difficult to know if I will because there are the variables of torch/gas/size or workpiece/melting point of filler to consider.
I think MAPP (technically now MAP Pro) is hotter than propane but could be wrong.
How much time did you spend cleaning the steel before fluxing? And how far around the joints did you paint the flux on?