Floor pan patch weld advice

[acealfa]

Hi,

While working on my car I've unearthed some rust holes on the floor pan so I'm after some advice on how to go about the repair. I've had experience welding but never done anything like this before.

What size thickness metal would be recommended? I was thinking 1.2mm.

My plan is to use a Dremel to cut out the rust in square sections then weld new pieces in from the bottom, I don't really fancy sparks flying about inside the car.

Would this be the best method of repair? I did think about just covering the whole area with rust converter then putting patches straight over the top but would this be considered a bodge?

Many thanks!

Edited by acealfa on Thursday 22 October 18:27

[RobXjcoupe]

Panel thickness is generally 0.75-0.8mm. Best repair is to cut out till you find good metal then do your best to spot in place then seam weld with no material overlap to stop damp getting between the two thicknesses

[Richard-D]

I joggle the edge then overlap by a small amount to make the welding easier. You'll also find it easier to remove the carpets (and seats probably) and weld from the inside. Welding above you is always difficult, particularly if you haven't got a lot of room to move (axle stands as opposed to a ramp). The prep takes a little longer but pays for itself usually.

Edit: Just to clarify, I'm not saying Rob's wrong just that the situation is rarely ideal and most people will struggle to butt weld thin sheet material.

[PhilF329]

I would definitely remove carpets and cut back to good metal. You will get a better finish IMO seam welding underneath. I often weld in repairs within the car but If you are joggling the edge you really have to seam weld underneath too. If you don’t you’ll have a potential water trap / anomaly that attracts attention at MOT time.

Make sure you are prepared for the heat / spatter / sparks and wear eye protection.

[droopsnoot]

PhilF329 said:

I would definitely remove carpets and cut back to good metal.

Absolutely, definitely remove anything covering the top of the floor, goes without saying.

I'd do the welding from inside the car, maybe you could arrange a tarp around you when you're doing the work to trap the sparks. I find it much easier to weld "down" than "up", but maybe that's just me. Also I have the advantage of usually working on bare shells, so less for me to worry about.

As for the metal thickness, use the same thickness that's already there. Once you've cut it back to a clean edge, it'll be easy to measure. Something relatively modern is maybe 1.0mm or perhaps thinner.

I'd personally try to butt weld, but a stepped edge doesn't have to be a bad thing and I do that in some places if it's more suitable. Put some weld-through primer on the mating faces before you weld it up, and make sure it's properly sealed afterwards. The only moisture you're getting then is condensation.

[acealfa]

Thanks for the advice guys. I will remove the interior and carpet once I can get the car rolling & probably tackle from the top. I’m going to have a practice butt welding some sheets together before but will drop back in with some pictures.

[omniflow]

Additionally, don't try to weld in a continuous seam, do lots of small welds that eventually join together. Put some copper behind to help remove some of the heat. You can get a piece of copper about the size of a mobile phone with magnets in the corners - only really useful if everything is flat, otherwise use flattened copper pipe.

Consider using 0.6mm wire.

If your repair patches are made from galvanised steel, don't forget to grind off the zinc layer.

[acealfa]

Thank you for the tips.

I've fast realised my current welder isn't up to the job. It was only a cheap one but even on the lowest setting just blows holes straight through even if I just tap the surface. I'm considering buying a Clarke 135te as it seems the next level up & allows for a lower setting.

[PhilF329]

Butt welding is tricky to learn so practice on some scrap metal - make sure surfaces are clean and turn up the wire speed. Might be worth having a small overlap as its a much easier weld. I would tack weld the patch and then seam weld in short sections, keeping an eye on sparks / heat build up etc. Keep a fire extinguisher, damp cloth etc handy unless you are really confident welding in situ.

[acealfa]

Just to give an update on this. I've learnt so much doing this that if I was doing it again I'd do a much better job & do things a bit differently. I got more of a feel for it as I went on. In the end I'd had enough working on my back with no space in a tiny garage that I just left the welds & didn't grind them down so doesn't look the greatest from underneath but it's no solid & passed an MOT so I'm happy for the time being.

[V8covin]

Not much penetration there, should have turned the power up a setting.
I would have done it in 1 piece personally

[acealfa]

V8covin said:

Not much penetration there, should have turned the power up a setting.
I would have done it in 1 piece personally

Any higher I was blowing holes but maybe something needed changing with settings because even when I was welding on test pieces it was building high welds if that makes sense.

[Evoluzione]

Your other problem is you've actually left the rust in, you can't weld to rust very easily. Cleanliness is very important when welding.
A patch lapped over would have been better than trying to butt on a floor pan.

[V8covin]

acealfa said:

Any higher I was blowing holes but maybe something needed changing with settings because even when I was welding on test pieces it was building high welds if that makes sense.

Has a similar look to when you try and mig weld and forget to turn the gas on

[sgtBerbatov]

I have/had the same thing with my Lada.

I went in to it with the idea to just cut the rust out and weld a patch accordingly. The problem I found though was that the metal near the rust, albeit was clean(ish), is weaker, as rust doesn't sit in one place. It spreads out. So I ended up cutting a bigger section out.

Like you I went to do a butt weld, but unlike you I made a hash of it. So I resolved to using a lap weld. Got a joggler, made a panel and sat it inside of the car. Popped some holes on the overlap, welded them and then welded around the edge of the panel. I then went under it and did the same there.

It's easier to do a lap weld in these situations I think, and unless it's a concourse restoration, a lap weld may have been better. But this was also the first time I ever welded.

This is the video of how I went about it, so you can see what I did wrong so no one makes the same mistakes: https://www.youtube.com/watch?v=4xfFw5Zymac

[acealfa]

Evoluzione said:

Your other problem is you've actually left the rust in, you can't weld to rust very easily. Cleanliness is very important when welding.
A patch lapped over would have been better than trying to butt on a floor pan.

I did realise i should have cut more out as I started but was a bit reluctant because of the shapes but I might do it fresh next year once I’ve had some more practice.

The gas I have was a disposable bottle from Toolstation so maybe not good enough?