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?

Metal : ?
Old on old ?
New on Old ?
New on new ?

All old. OSHPO first then weld thru primer.
New just clean with paint prep and weld thru primer.

My .02c for tonight.

Scuff to clean metal, then phosphoric acid, let dry...then clean w/ wax grease remover, then epoxy prime, let dry...plug weld, scuff away slag, grind if needed, then epoxy prime the exterior of the new seam, add seam sealer... then the top of the sealer will get epoxy primed again before paint, of course.

There's more than one way to skin a cat but I've read at least a couple of articles that compared weld thru primer with just using epoxy primer.. and the EP was the victor. So, anyway that's the technique I favor, one less item to buy, if nothing else (I've paid good money for Weld Thru primer that wouldn't spray too), YMMV.

Definitely no expert here but I lap welded my front half of floor pan to the original rear half and worked real well for me. Weekend warrior in on a fixed budget quality. First grind the lap weld area the best you can to remove any rust areas. I like to use 120 grit roloc sanding discs on an angle grinder and wire grinding wheels. For the more stubborn pitted rust areas apply Ospho (multiple applications) and brush with a small dense wire brush between applications. Rinse with water and dry immediately with a heat gun. Locate and drill your holes for puddle welds over good rust free metal only then apply the weld through primer to both panel surfaces prior to welding. Following welding I apply POR-15 with a brush to drag the sealer into the seams. When cured, use a seam sealer to close off any gaps in the seams using caulking gun or putty knife application. By the way, I like using cleco temporary fasteners on lap welding to hold your panels tight to each other before doing your plug welds. They make it way easier, cleaner results, and stronger puddle welds.

This was my process too. Only thing I'll add is that you need to scratch away the primer inside the plug holes with a scratch awl or screwdriver before welding. That's true even if you use weld-thru.

I started my resto using Upol copper weld-thru which is really nice stuff, but I noticed it was burning back from the welds just as far as epoxy so I switched to just using epoxy to save money. More black soot with epoxy than Upol but otherwise seems to be a similar end result. I also followed the welding by brushing the edges of the seams with reduced epoxy to let it seep into the joint.

UPol high zinc does not burn back much it at all in my experience, good product.Click to view attachment

That's interesting. I am a novice welder, and I always had less than satisfying results with the myriad of weld-through primers I tried. I just assumed weld-through was meant literally! I'll keep this in mind for future projects.

I use the same U-Pol weld thru primer.
But it only goes under the lapped or overlaid metal. Over-spray wiped off with lacquer thinner.

I scratch out the to be welded rosette hole with a blunt/cut off drill bit.

Weld through primer is a misnomer in my opinion. Maybe "weld nearby" would be better.

Much better results if you remove it from where the weld bead will land.

Maybe it does not burn away from the surrounding areas as much as other paints or it leaves behind zinc around the weld area?

Heating up zinc to a vapor and breathing it is not a good idea health wise so take precautions.

John

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.

Ja, me too. I've had several cans go bad on me that have barely been used

At least I know I'm not the only one.

Weld through primers I have used always contaminated the welds if they get into the welding puddle. I masked off the areas with tape where the actual welding will be then sprayed on the primer just to have some sort of protection in areas that may harbor rust in the future. Make sure the back side of the weld has no primer either for same reason.

I've been following a similar procedure to most here. But, I've been using POR Metal Prep on the old metal after media blasting. I contacted POR and asked them for their recommendation re wax and grease remover prior to epoxy. Their response was that wax and grease remover might adversely affect the paint adhering to the metal. They said the epoxy (or POR paint) should be applied within 48 hours of the application of the metal prep (once dried, of course), and if outside that window to reapply the metal prep. Then apply the epoxy primer (POR paint likes to adhere to any flash rust that forms but they said for epoxy primer to either leave it or possibly sand off any flash rust, they weren't sure which would be best in the case of epoxy primer). So, I've been reapplying my metal prep (and neutralizing with water, of course) the day before epoxy priming (so the metal is completely dry) and then just blowing off the area to remove any loose, chalky zinc coating. It seems to be working very well. This is for structural repair areas and not on the cosmetic parts of the car. YMMV.

Regarding weld thru, I started out using zinc and using a broken/flat drill bit to clean it from the plug weld location, but, despite doing my best to remove it from the plug area, I didn't like how it welded at all. I switched to UPol copper and I don't even remove it from the plug weld location. It welds thru great IMHO. I only apply it to the projected 'heat affected zone' of the weld area. Apparently the theory with zinc weld thru is that it's proximity to the weld (where it was burned away during welding) is enough to inhibit rust even in the burned area. I'm hoping the copper works the same way.

FCA Explains Ban on Weld-Through Primer

Here's a link to a discussion I had a couple of years ago with 962Porsche on The Bird regarding weld thru primers: http://forums.pelicanparts.com/paint-bodyw...hru-primed.html

Thanks, everyone.

The ban link is a good one.. Remember steel melts at 2750 a d no primer in the world can withstand it so naturally as it degrades in the weld area(HAZ) then it will contaminate the weld area..typical carbon steels have a range from 1008 to 1095 with carbon added to the iron to make each grade of steel.. The amount of carbon added is approximately .08%-.95% respectively.. Now if you add another carbon source then it changes the steel dramatically thus potential embrittlement to the steel.. The same thing can happen just by using some layout pencils and writting utensils..