Consensus on Lap Joint Weld Prep? Options

[bkrantz]

So based on experience and expert knowledge, what's the best way to prep metal for lap joints, especially with plug welds? No paint or treatment? Ospho? Weld-through primer? And what about seam sealant after welding before paint?

[Superhawk996]

Just a generalized statement.

Avoid lap welds if you can. I know they are easier than butt welds but they tend to lead to more corrosion in the future since they tend to trap moisture even under the best sealers. Additionally, if there is any rust still caputured within the lap joint, it is just a great place for it to simmer and fester into future rust. Tin worms never sleep.

I do realize lap joints cannot always be avoided and that they exist in the factory sheetmetal.

If lapping, make sure the direction of the lap joint is such that wind, and water spray from the road cannot be forced into the joint.

Example: front floorpan area facing forward. You want the front most piece of sheetmetal UNDER the rearward piece. That way, wind, water, oil, etc., flow over the front metal and blow off the lip to the rear metal. You don't want the contamination forced into the joint by air pressure.

I would absolutely seam seal a lap joint due to it's tendency to hold moisture.