Just curious how the pros deal with lap seams such as where the inner fenders and braces meet the firewall. Even the "rust free" cars seem to have plenty of rust in these areas when the spot welds are drilled and the metal is peeled back. I can't imagine anyone goes through the trouble of deconstructing these areas to treat and weld back up. They would never look right.

If the metal looks good, I generally just coat with epoxy and use seam sealer in the appropriate areas. But that doesn't change the fact that there is unseen rust.

Like Neil Young used to sing
Rust never sleeps

the best you can do is
soak it The best you can
with a rust inhibitor like ospho
And seal it up with epoxy primer.

So are you saying that you open up all the seams, treat them, then weld them up? Or, are you treating the visible rust, then epoxy?

Treating the visable rust the best you can
You aren't going to open it
Unless you are replacing.

I have a Fluid Film rustproofing gun, I just spray into the seams - other than separating the metal (which I doubt anybody would do), forcing it in with the spray gun is what I have done.

What are you spraying? I am reluctant to spray a phosphoric acid product in there because it will cause problems with the paint if the residue is not cleaned off.

I don't know how well it works but they've talked about Machinist Mate Rust Freeze on "Performance TV" on Velocity.
http://www.ispot.tv/ad/AoX0/machinist-mate-rust-freeze-loves-corrosion

I believe they said it could be painted over.

I used a product from Permatex (IIRC) call Rust-Stop(???). Not sure anymore, I'll have to look on the can. Stuff worked great. Very thin also so it could "flow" into seams. Turns black and you can paint right over it. I'd highly recommend it.

oh heavens no, i would hate to spray in any type of acid, it would just sit there and fester and rot it from the inside out...fluid film is a non evaportaing lanolin oil used as a rust preventative....google it...every year my friends and i have an annual rustproof our cars weekend on my hoist - we all chip in and buy a few gallons of the stuff and drown the undersides of our cars...another substance i like to rustproof with is linseed oil - it leaves a nice hard candy coating tha does not come off easy...

oh heavens no, i would hate to spray in any type of acid, it would just sit there and fester and rot it from the inside out...fluid film is a non evaportaing lanolin oil used as a rust preventative....google it...every year my friends and i have an annual rustproof our cars weekend on my hoist - we all chip in and buy a few gallons of the stuff and drown the undersides of our cars...another substance i like to rustproof with is linseed oil - it leaves a nice hard candy coating tha does not come off easy... [/quote]

OK, I thought you were just using the gun for spraying something else. I use Fluid Film on my plow truck. I am thinking more in terms of prepping for paint, which obviously, FF would not be suitable.

I was curious about this topic after seeing some high dollar restorations that started out rusty and were not completely dissasembled so they undoubtedly have rust hiding in the seams.

Some big $$$ jobs get dipped, and E-coated...

I would bet that dipping doesn't get into the seams completely.

3m Rust Fighter works well too. Creeps in like fluid film does.