How important is prep (actually) ?

[Face]

I'm being honest. I'm a beginner. I'm welding mild steel. It's fresh from metal yard. I it gun metsl colour not shiny. It feels like 6013 is happy to stick to it and Penetrate it. Now I know its better practice to prep. And I haven't tested my welds to the n'th degree but if it can blow a hole is it not getting deep into the metal? Do you really have to polish every weld before striking an arc?
Is it even necessary ? Pls forgive in advance in lf I have offended anyone

 

[Barking Mat]

Yeah, it makes a difference, you can weld minging rubbish without cleaning up, but you get a better and stronger weld if you prep it.

 

[cocker]

Do as you like , if your boss is happy - crack on .

 

[Face]

Yeah, it makes a difference, you can weld minging rubbish without cleaning up, but you get a better and stronger weld if you prep it.

Does it still work though?
I know its taboo. Maybe not on a car or a house steel etc. What is the percentage of strength loss on weld strength if welding onto millscale for instance? If someone says it doesn't work I will take that advice. If someone says yeah it still works ill take that advice too. I get the impression that it at least works with my welder. Maybe other welders wouldn't. I mean is it known that rust helps draw current so facilitates arc depth etc. Or is it that it Sheilds it?

 

[fizzy]

It takes a few seconds to hit it with a flap wheel on a battery angle grinder.
Mig likes clean metal - with arc not needed so much but still helps.
As a beginner it is important to see what you are doing so clean metal really does help.

 

[Memmeddu]

In Tig welding is crucial.
Dirty stainless would not weld at all, the arc would wander everywhere, uncontrollable.
Aluminum will become like a sponge.
It will weld but with very poorly results, cranking up the positive balance you might be able to make impurities to float on the weld but it's not ideal.
Sometimes happens to have perfectly clean surfaces but when you start to weld you'll see a lot of black stuff coming out from the weld.
Problem is that many times you have to stop grind and wire brush again multiple times because this will form a thick and refractory layer of oxides and you are doing nothing.

Mig welding is kinda optional if you have enough current you're welding till you have metal below.
Just be sure to have not oil or grease beneath and it should be fine.

Stick welding is optional too same thing as long as you have not grease or oil it will burn through the oxides mill scale or paint resulting in an acceptable weld

Important to say that's from my experience on real world jobs but none was coded and everything change upon materials or what you have to achieve with your welds.
Cleaning make an huge difference for both mechanical properties and ease of performing the weld joint.
90% of the time well worth to spend some time cleaning.

 

[chrismac]

I am sure your talking stick/MMA here, it all depends hat your doing, smallish new flat, bar, angle, box, channel, you get the drift, I wouldn't bother, if heavier stuff the prep, not just a clean up but a bevel on the mating edges is there to get a better weld with more bite (poor description)
Dirty, rusty metal, as an old guy at work used to say, "you'll be chappin like a hen" to get the rod started, if you dont clean things up a bit.

 

[Face]

It's stick welding newish steel.
I don't think prep is ever time wasted. It usually makes for best results. I do find myself making quick follow up hits and not always brushing or brushing but not being sure if I've done it enough. But laying on plenty of weld and believing I have a strong ( enough) joint. Certainly seems to have worked. I'm no pro however just a d.i.y er on my own garage shelves