Hi,

I am looking for definitive guidance and perhaps a pointer to a thread on this topic.

We have finished all of the welding on metal that was primed with weld-through. We will now grind the welds. Is this the correct thing to do in this order?

Zinc Primer
Weld
Weld some more
plug up the holes with weld or epoxy (screw holes and small gaps)
Grind the weld and expoy smooth (smoother)
Treat the welds with 3M weld putty
spray another coat of Primer
Coat with POR15

Do I have this right?

Flame ON!!!!

POR15 works best directly to metal, as its a rust converter and is very "sticky".

Most people no longer use POR15, especially if its on bodywork. If its a product you want to use (And I wont fault you for wanting to use it, I have it on my floorpans) then I would do your zinc primer, all your metal work. Por 15, then any sealers, primers, and topcoats.

I would not use POR15 on bodywork. I would however use a primer sealer. \

Do all your metal work.
Once done, clean your body and apply a primer sealer (This is typically a non high build primer). Tint it or have it tinted in another color than your primer fillers. This way when you get to it, you know to stop sanding and add more primer filler. At this point you can apply a high build primer or bodyfiller, however note you do not want to break through your primer sealer as that is your moisture barrier. I would reccomend going with a high build primer directly over primer sealer, then bodyfiller, then rough sand, followed by another layer of high build primer and guide coat. When its all said and done a final layer of primer blocked to give a consistant base color (Tinted on the greyscale to match your final color), then final paint.

Lots of work and materials to do it right.. Theres a reason this type of paint job costs 10k... There is typically 3k in materials!

I know the feeling. I often worry when I cannot easily get at the backside of a weld. Anything with burning and soot isn’t a substrate for paint. So I grind and sand and then coat with epoxy. I use the PPG paints so for me it is DP-40 or some other color epoxy primer. Epoxy paints are waterproof to both liquid and vapor. Sandable primers and paint coats are more permeable. So I make a litttle steel mummy with that.

Thank you Andy!

To be clear, this is for floor pans and hell-hole. The body is amazingly absent of any rust other than a few pockmarks.

I think I will POR15 all of this metal - I sprayed it once with weldthrough to keep it from tarnishing in the humid weather we were having, any trouble going right over the weldthrough with POR15?

Respectfully,
CTC

Weld. Grind. Etching primer or epoxy sealer.

The etching primer has acid in it and puts a fine etch into the metal which makes a good bond to the metal. Etching primer is not water resistant. It is porous. It will slow down rust forming, but not stop it in wet environments.

The epoxy primer seals the metal. You will want to buff the metal with 400 grit paper before putting down the epoxy, as it is not self etching. I used epoxy on my car instead of etching primer. Epoxy will stop rust from forming, and encapsulate flash rust to keep it from spreading. POR15 is a type of epoxy primer sealer, but there is better stuff on the market these days at any auto paint supply place.

Zach

POR15 is not a rust converter, it's a rust encapsulate/paint. Por15 makes a product called metal ready that is a rust converter/etcher.

There are lots of threads on here complaining about POR15 products, but there is also a lot of misinformation like this that leads to their products being used incorrectly.

I used por15 on my floor pans 10 years ago, and it still looks like the day I applied it. But I also followed the instructions which include removing as much surface rust as possible manually, cleaning the metal (I used the Por15 Marine Clean product), applying Metal Ready metal prep and then applying the POR15 paint.

If you are going to use the product, take the time to do it right so your effort isn't wasted.

POR15 goes over properly repaired bare mental only.

agree with rjames and Tdskip. POR15 works great for what it is designed for, but don't apply it OVER weld through primer. apply directly to the metal.

also, if you want to topcoat the POR15 you should do it within the timeframe specified (like 60 minutes IIRC) otherwise regular paint/topcoats do not stick well to cured POR15

Por15 is garbarge. If you have real rust, cut it out and replace the sheet metal. If it's truly surface, then treat it right with sulfuric acid (such as Oshpho) and seal it with a quality primer and paint. Not that Por15 is all bad, but ignorant people use it as a band aid thinking it's going to magically cure rust. Why would anyone think that covering over rust is a good idea?

Well the whole sales pitch is based on painting over rust. Which you can do. It seems closer to super glue than to epoxy. At any rate there are all sorts of things you can paint over rust. With predictable results. Why is it that Neil Young was the one to pin point the problem?

Ospho is phosphoric acid, not sulfuric. With all the rust potential 914s have, these cars barely exist in our time/space continuum. I can’t imagine how quickly they’d disappear in sulfuric acid. But I now have an idea for a YouTube video...

Btw, I’m not a fan of POR-15. I used some on my mustang and it peeled off in a few years. Despite using their full prep system. If you’re hellbent on using a product like that, try Rust bullet. Much better results.

Oshpo works Ian. Yes phosphoric (my bad for saying sulfuric). It only eats rust, not good metal. I'm a fan of getting rid of the rust, not painting over it. And while I love what Oshpo does, I'm not suggesting it's a cure all. Get rid of the rust. But if it's surface, then phosphoric acid is great.

I just came inside from treating all the internals on my bare tub with Ospho/phosporic acid (I'm actually using Klean-Strip brand because it is cheaper and more readily available). I'm also a vote for rust converter but am using a belts and suspenders and another belt approach. All internal frame members get a phosphoric acid treatment followed by Eastwood Internal Frame Coating which is a rust encapsulator, which will be followed by cavity wax which is another rust encapsulator that I've had very good experience with. My understanding is that a rust encapsulator simply seals the rusted area off from oxygen so the oxidation process is halted. That means the encapsulator works only as long as it provides that perfect seal from oxygen. So I think it makes sense to get rid of as much rust as possible either through removal or chemical conversion first.

For the rest of my metal work, the process I've decided on is to:

1. Grind welds
2. Cover butt welds with this DTM filler that @mb911 turned me on to:
   
   https://www.amazon.com/gp/product/B000P70VG...=UTF8&psc=1
3. Spray the whole car with PPG epoxy primer
4. Then follow with whatever surfacers, fillers, or top coats as appropriate.

Ospho is a great treatment. Probably better than weld thru primer, but I don't want to ignite a rusty debate