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mk2marty

Rust repair - butt vs lap joints

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So there's a hole in the floor of my Escort. Would it be best to butt weld the floor repair section in, or lay it in from underneath and lap joint it - ie. weld it from the top? Bearing in mind that there's also the lower pillar and sill sections to be welded in too.

Snapchat-893378906.jpg

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Why not both?

It looks like you need to repair your Inner sill and floor section before you add the pillar and outer sill over the top.

Seperate the floor pan from the inner guard at the seam in the left of the picture, overlap and plug weld it. Overlap the flat part of the floor pan.

Butt weld as the factory have done down the left edge of the pillar and butt weld the sill at the right hand side.

That's how I'd approach it.

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IMO you only want butt welds where you joining mid panel to mid panel so its invisible, basically where you are replacing rusty sections with new panel sections.

Use lap joints (with plug welds) to replicate the factory spot welds where they would have existed before. Lashings of seam sealer to keep the moisture out of the overlap.

As Valiant says, there is a bit of trick to thinking about the order of assembly re getting the grinder or mig torch in especially in the inner corner of the floor - eg maybe butt weld the floor section in, build up the inner sill to the new floor datum, build the outer sill (butt to the fender and sill, plug on the bottom lap) and plug weld the floor lap to the new inner sill last.

 

Also that patch looks pretty rusty - best to cut more out untill you get to good metal, as its easier to make a bigger patch now,  than try to weld to rust and then redo the repair in a year when the weld blows out.

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I've done both floors and sills in my mk2 vans and did them a little differently each time. But each time I built them from the inside out. Ie, built the floor and inner sill so it sits flush against the original sections and outer sill. Cut out the outer sill and pillar, and repair them. Lapped and plug welded were the factory joins were, and butt welded the other.

Another suggestion would be to cut your floor out even further towards the gearbox tunnel. Slice the floor where it runs along the middle of the chassis so half the original floor is covering the chassis and the other half needs new metal. This is a very strong area to weld because it will also be plug welded to the chassis and it means your join going north south wont be seen from underneath due to the chassis being in the way. Keeps WOF man happy.

Keeping it in factory placement and less joins, I would also suggest cutting out more of the floor going towards the firewall. The floor overlaps the floor slightly and is plug welded. Take that out and replace. You will only need to lap and plug weld this section of the floor on. Again, it saves making another join. If you did it this way, you will only have the one join showing going from the sill to the gbox tunnel

Hopefully the above makes sense. The photo below will show you what I mean.

post-17673-0-07548900-1402919553_thumb.jpg.bbba2a64fa7c9dd6eb4252a481cc8529.jpg

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All of the rust repairs i've done on current car are rusty lap joints. So I would lean towards butt joint as there is nowhere for water to get in. Butt join is more work, as fitment has to be pretty close. But i'd consider it a better method. Everyone is going to have different strokes though. Whatever you do, coat it all to stop moisture getting to it again. And as someone mentioned, if wanting long term fix, you'll need to cut more out till there isn't rust on the edge of the patch. Cut it out till you have good solid metal to work with.

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Oh yeah and something i've done on the "big chobs" is cut out a smaller section, and replace that, even if the edge is rusty. Then cut slightly into the new repair, and extend the hole to remove more of the rusty area. If that makes sense. Rinse and repeat. Means that on areas where getting the original shape back might be hard, it's easy. As you're only replacing small bits at a time, while the cars shape doesn't change as the previous repair holds it together. I've had to do this quite a bit, as doing it in one piece would be near impossible to recreate the original section as it was.

 

tl;dr: don't always try replace a whole area in one hit, small bites works well.

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I'd second what bling said, lap joins are inferior by nature so if you use them, use weld-thru primer and seam sealer to exclude air/water and thereby delay the rusting process. They are much quicker/easier than butting in though. 

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Cheers all for the feedback, it's a huge help when the holes in the car just keep getting bigger, and the motivation gets lesser...

As much as I'd like to make the repairs as invisible as possible, the replacement panels limit that a bit. What I'm thinking at this point is to cut the hole larger, drill the spot welds on the inner sill/firewall and use the replacement inner sill panel and butt it to the rest of the floorpan, and plug weld it to the sill and firewall in the same place as the factory did. 

 

Snapchat-1884628526.jpg

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Good plan. 

If you draw/ mist some primer round the new panel onto the car and through the hole onto the panel will give you min/max cut lines on both and a better idea how much panel you have to play with/ how much more you could chop out.

You could also self tap the panel in (temporarily) and run the cutting disc between the two lines above to make a nice matching seam to butt weld.

As long as you have the panels nicely lined up, take your time and some flap discs theres no reason that seam would be visible when its all finished

 

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