Welding/ bondo/ moisture

[Olav]

Have seen a lot of photos where the welding seam is built up by lot of small tacks to not warp the sheet metal. I also do the same. I have seen a lot of builds from amatures and pros that dont have the weld fully covered by tacks. Will it be a issue with moisture coming thru the pinholes/ holes from the inside and into the bondo/ filler and create rust? I know some are using epoxy but this is not always. Would it not be better to use lead before bondo/ filler?

Olav

[64SS327]

You would be correct about wanting to fully weld a seam. Even if the inside of panel is painted there could be a possibility that corrosion can start from the inside out.

I see many people applying body filler directly to bare metal. The metal should be sealed with a quality epoxy primer first even over patch repairs. Fillers are good enough now days that lead is a thing of the past. People use it but acid can get trapped in the filler from the flux causing issues down the line.

[jlcustomz]

Even polyester fillers in good brands such as evercoat have some moisture/ rust resistance to them. But the preferred filler for a first coat against bare metal (without epoxy primer first) would be USC all- metal or equivalent, which is a fiberglass based product meant for areas such as welds.

I have seen some custom widebody stuff done by body shops where they tack seams & apply fiberglass resin/ mat to each side. May be great for moisture issue, but not how I'd want to be caught doing it. I prefer more time spent in decent metal fabrication & less band aids.

[Jetfixr320]

I tack weld mine to fully cover a weld joint. And still use good 2 part epoxy and duraglass on the inside to seal it from moisture.

[Death trap]

I agree with all above that all the seems must be fully welded if the seem goes threw the middle of a panel. The catch is it is super hard to fully protect it every were. The factory dips the bodies after welding to protect it inside and out. If you mig weld you normaly grind both sides of were you are welding to bare metal to avoid contamination of the weld. But you can't always clean up and protect the bare metal on the inside and if you leave the paint or e-coat on the inside, normal mig welding will burn the coating and it is still not protected. There are so called weld threw primers that are better then nothing but are more like weld near primers cuz they'll still burn. I have just recently started silicon bronze mig welding my joints that run threw the middle of a panel and like it alot. It welds at a colder temp so it doesn't burn the protective coatings as much plus less heat means less chance of worping.

I only epoxy prime projects were I will have bare metal exposed for a long period of time, mainly larger projects. If I use body filler and can get paint on it in a fairly short amount of time I feel confident it will hold up and have never had a problem like that. Modern body fillers are way better then the used to be but all are porous to some extent (besides fiberglass). But the fiberglass fillers do not finish easily and are generally harder to work with. Epoxy primer also doesn't sand well but it does protect well. I even think most non epoxy primers are porous too and absorb moisture to some extent. Just some food for though