How bad are a few pinholes

[80sz]

I did a patch and put a light behind it. I have some pinholes in the weld line. How worried should I be? It will have to be gone over with a little filler. I dont want to over grind and make the metal thin but also want a quality job. Should I leave them or tack them in. They are very small holes. I was going to undercoat the back side and call it done . What do you think?

 

[dsbratton]

That is what filler and seam sealer is for. If you choose to tack, I would use a copper backing so you don't make them worse.

 

[80sz]

Thanks. I needed to ask for Im a first timer at it and learning as I go

 

[CasperCasper]

Use a waterproof filler

 

[jthomas]

Pinholes are inevitable when you mig weld. You can use any high quality filler, but like you mentioned the most important step is to coat the backside with a seam sealer, undercoat or other product to prevent moisture from coming in from behind.

 

[Twisted_Metal]

Some types of undercoating (beeswax type) can reek havoc on the paint if it can soak through the pinholes. (The stuff never really hardens.)

It creeps under the paint and lifts it. This forms a bubble that looks like a rust, but isn't.
I've got a couple of those bubbles on my car.

A seam sealer which sets up hard would be a better choice if you're not going to weld the pinholes.

 

[twozs]

Pinholes are inevitable when you mig weld. You can use any high quality filler, but like you mentioned the most important step is to coat the backside with a seam sealer, undercoat or other product to prevent moisture from coming in from behind.

Not quite true . In mig class we had to make a cylinder out of flat stock and it had to hold water , then it was sealed on both ends with water in it and had to hold . If your welds are boogered up , something is up with the gun setup ( heat , gas or wire speed ) . Pin holes equals a weak bond .

 

[jthomas]

Not quite true . In mig class we had to make a cylinder out of flat stock and it had to hold water , then it was sealed on both ends with water in it and had to hold . If your welds are boogered up , something is up with the gun setup ( heat , gas or wire speed ) . Pin holes equals a weak bond .

I don't disagree that you can mig weld without holes, but it takes a practiced welder, a high quality welding unit, all the right settings and so forth. In the application we are talking about (body work), you will get pin holes unless you are exceptional. Unless they are visible holes, very small pinholes should be expected and sealing from behind is best practice. Clean, clean, clean everything before applying sealer to the back (when possible) and same for the front before applying filler. Do that and it will last a long time.

 

[twozs]

I don't disagree that you can mig weld without holes, but it takes a practiced welder, a high quality welding unit, all the right settings and so forth. In the application we are talking about (body work), you will get pin holes unless you are exceptional. Unless they are visible holes, very small pinholes should be expected and sealing from behind is best practice. Clean, clean, clean everything before applying sealer to the back (when possible) and same for the front before applying filler. Do that and it will last a long time.

I worked at gm as a mig welder for 10 years and probably miged about half a million or so cars ( probably more ) before ever learning a thing . They stuck a gun in my hand and said " here , here and here " ... if your not ready ...they hated holes as it weakens the weld . You learned how to fill them in ( on a moving car no less and with less than 60 seconds to do it ) .Good luck OP , keep at it . I would fill them .