Rust Protecting a floorpan: Anti-rust paint before or After?

[mashedpot8er]

Hi there. I've started to weld a plate into my car floorplan and have run into a few questions. I'm a complete beginner to having a go at this sort of thing, so sorry if the questions are a bit stupid I've cut out the rust and given it a going over with a whire wheel, all good so far. Here's a piccy.



The little holes numbered 1 need cutting out, but has the same issue as number 2. How can I protect the metal before I cover thing up with the plate ? You can see a flush bit of plate that the new plate will sort of sit on in No.2. I've got dinitrol rust converter to spray about, but I'm not sure if it's best to spray the car side with it first, then weld over as normal. Or to weld the plates in , then spray it over with dinitrol and hope I can get plenty in the gap between the touching bits from the underneath afterwards. I'm probably going to weld the plate from the bottom where I can get to it afterwards too. Then rust convert / prime/ stonechip etc.

Hope you get what I mean, I do ramble on a bit. Here's where the holes in question are. I did wonder about weld through primer too, but I don't know if it will protect it enough once it's all sealed up.

Thanks for any help.

[theblackbox]

Personally if its a car you will keep cut out and replacing it is the route to go. Cutit right out to clean metal then get some cereal packets hold underneath draw inside, cut out check for fitment then transfer to metal ideally of the same thickness that's not got any coating on, weld in, flat welds down beat to shape. That's how I do all welding on cars patching doesn't really last I don't even really like a lap weld even if you do lots of rustproofing which tbh most will burn away/stink/make your welds look very poor you'll get it spitting. Obviously the above is just my advice with my limited experience it is hard butt welding pannels but its worth it when everything is perfect/how the manufacturer had it originally.

[cobblers]

As above, if you plan on keeping the vehicle for some years butt weld it where possible as it doesn't leave a moisture trap. It's really no harder to weld, it just takes more time to make the patches.

To make perfect sized patches very quickly, cover over the hole with strips of masking tape, then mark round the outside with your mucky finger. Peel off the tape, stick it to your sheet of metal and then cut round the dirty line:









This took literally 15 minutes from finishing cutting the hole to the first weld.

[prey]

the masking tape is a bloody brilliant idea lol.

as above - don't weld plates over stuff, cut it out!

[m1keh]

Masking tape is a good idea for a flat patch. For a curved one it might not work so well, as it'll tend to straighten the bends and be too short.

As far as the original question goes, you might find the rust converter will cause a bit of weld spatter, but it might be better to convert the rust and then buff back to clean metal with a prep disc or similar. However, if you're butt-welding it doesn't matter so much if you do it before or after, as almost all the surface area is still exposed afterwards.

Personally I find butt welding a lot harder than a 3-5mm overlap joint, as I tend to blow holes. I need to try it with a heat sink behind though, as that should help. I'd have said the 2mm ish gap you have at the end in the picture would be a bit big to butt weld, but maybe it's all about welding technique. I always use short-pulse welding when doing butt welds.

[joostvdw]

Jul 19, 2013 9:50:33 GMT m1keh said:

Personally I find butt welding a lot harder than a 3-5mm overlap joint, as I tend to blow holes. I need to try it with a heat sink behind though, as that should help. I'd have said the 2mm ish gap you have at the end in the picture would be a bit big to butt weld, but maybe it's all about welding technique. I always use short-pulse welding when doing butt welds.

If you're blowing holes then your power settings are too high or you are staying in 1 place too long. With the proper settings you can weld .7mm sheet metal, even with .8mm welding wire!

As for the rust, I agree with cutting it all out, even if it's only surface rust. You can never really kill rust, not a single rust converter or chemical will do that, you'll have to cut it out. Of course, if you just want it to get through the MOT for the next year you'll be fine with overlapping and covering treated rust, but even then it's a nice feeling knowing it's all gone

[nomad]

An idea of what you could do
retrorides.proboards.com/thread/88892/1972-hillman-hunter-estate-lives?page=8#scrollTo=1812736


I'm liking the tape idea

Jul 19, 2013 9:50:33 GMT m1keh said:

Personally I find butt welding a lot harder than a 3-5mm overlap joint, as I tend to blow holes. I need to try it with a heat sink behind though, as that should help. I'd have said the 2mm ish gap you have at the end in the picture would be a bit big to butt weld, but maybe it's all about welding technique. I always use short-pulse welding when doing butt welds.

For butt welds, try short bursts - leave a gap - do next burst
then go around again when the welds have cooled to fill the gaps, and again ... until all gaps are filled

A heatsink is great if you have a gap between panels but you should be able to but-weld ok without - but only if the patch is a tight fit.
Try weaving the tip across the joint a little so you bridge the 2 pieces together

I use an offcut of 15mm copper central-heating pipe as a heatsink.
Flatten 1 end (other end is the handle ), bend into a swan-neck shape and you've got a very handy tool for pennies
Worked well on the lower left corner of my patch in link above where I had a gap
I find it best to hold the copper behind the hole, run a short bust then let the weld cool a little bit - don't try to fill it all in 1 go.

Have fun

[mashedpot8er]

Wow, thanks for all the help and suggestions, brilliant stuff. Love the masking tape and mucky finger technique, missed it for making this patch, but will definately use it on the next one.

Seems general concensus is butt weld with short bursts leave a gap - repeat until all welded. Seems like it shouldn't harm the weld too much if I squirt on some rust converter first too.

Thanks so much for your help, I'm going in !! ( will post up more pics of how it turns out.)