good weld-thru somewhat permanent primer?

[Ochre]

Curious about this as it pertains to some rust repair I'm embarking upon...

as far as I can tell, the factory didn't bother to, or did a piss poor job of finishing the backside of the door striker panel, and the area between the back bed floor and rear cross sill. This is why I theorize that there's always, if not almost always, surface rust in these areas, and the back sill area rots out (besides the fact that the rear hatch area leaks badly, and the "sandwich" of metal traps water and causes rust in that area). I have a few thoughts with regards to semipermanent fixes in these areas:

It seems to me that these areas are particularily nasty to deal with in a few respects:
1) both the striker panel/bedside and back floor/rear sill sandwich metal to metal, trapping dust & moisture that happens to get in there
2) both these areas need arc & spot/plug welds to join panels
2a) heat from welding damages most automotive finishes (apply before)
2b) por-15/rustbullet/etc aren't billed as weld-thru
2c) because the metal "sandwiches" together, applying finish after welding is difficult/impossible
3) weld-thru primers are just that, primers. Most primers will rust thru given a long enough timespan, and/or a difficult set of environmental conditions (high humidity/salt like here in MN)

Given those constraints, a few thoughts spring to mind, pre-paint:
A) Use a non-heat method of joining panels (glue) - makes me nervous
B) weld unfinished panels together, find a method of spraying an anti-rust finish (like thinned por15) into the unfinished panel and sandwich
C) spray anti-rust compound into area in vicinity of weld, weld plug weld and hope heat cures compound properly - also makes me nervous as even the coldest process for welding (mig) is still way higher temps than any rust compound I've seen takes to accelerated dry
D) seam-seal edges of sandwich post-welding - could trap moisture, or let moisture in thru a pinhole over time (also messy and looks like ass upon close inspection)
E) perhaps my best idea so far, finish panel but grind spots to be plug-welded and spray weld-thru primer on those areas (good assuming no interaction between the different finishes, and that the rust doesn't get under the anti-rust compound

I'm curious what other members have to say about this...I'm looking at investing tens, probably hundreds of hours into repairing the rust on my blazer, which I intend to keep for some time. The last thing I want to worry about is...drumroll...fixing more rust! I'd rather worry about fun things...like more horsepower and more gears and which rims look the best, etc.

It seems to me that these are key areas, along with the rocker boxes, that seem to rust out to some extend on every blazer/jimmy, regardless of location due to inherent design flaw. Mostly, it seems to me that little or no thought was given to where water would flow, settle and escape (or not)

Naturally, the thing to do would be to correct these design flaws, with some corrective surgery - but most would prefer to keep the stock looks as well as the inherent difficulty of corrective actions (most require extreme welding, done incorrectly this warps panels, putting us at a worse position than simply square one). Plus, if done incorrectly (and only time will tell) the additional weldment will make panels that need to be repaired a second time harder to remove and fix.

So, I'm curious as to some member input on good weld-thru primers that can withstand the test of time, to keep my ride rust free for as long as possible after my current battle with rust. Is that too tall of an order?

[vtblazer]

I've had good luck with SEM 39783 in the rattle can version.

On the can:
"PROVIDES CORROSION PROTECTION WHILE REDUCING DISTORTION AND WELDING SPLATTER"

[Huck]

Metal on metal is a problem no matter what you do. The lower door edge is a perfect example--a little pocket that will rust thru over time and no easy way to prevent it. There are "weld through-rust preventative primers" that are ok except where the weld burns and melts the metal. MY 2000 Tahoe and Silverado started to show bubbleing on the inner door bottom where the skin wraps over the inner shell---even though the metal was primed and sealed before painting!!!
I cover the back of rockers and the inner rockers where the welds will go with weld through primer then coat the rest with POR15 or other sealers with the hope that by the time it rusts through again, I will be long gone or the truck/blazer will be long gone. I blow out those pockets between the bed floor and the rear floor support with high pressure air--let dry well-then slop in POR15 to seal it a bit. Will let you know when it rusts out~~~hehehehe

[c-1072]

i think the only way to remove all the rust you cant see is to dip the tub . as for keeping it away i have been trying on my cars for years . I was thinking about some of those weld through primers also , but dont have any expierence with them either . seeing how i split my windshild frame sandblasted it and want to weld it back together .

[raggedjim]

I've been worried about the same thing. I'm also using SEM 39783 in the rattle can version before welding. But I was worried so after welding I sprayed everything with a real thin (wish I could remember the name) zinc-phosphate coating similiar to the coating POR15 sells as the pretreat for their POR15 rust encapsulator. Then I resprayed everything with a zinc rich primer.

Will this keep rust from coming back? I don't know. It's frustrating doing all this work and not having a good idea how long it will last...

I know I'm not capable of removing all the rust (it's gotta stop somewhere...). I just want to get it as good as I can and then hope it last for awhile (15-20 years or so...) then someone else will have to maintain it for a while! I think I have a responsibility to keep my trucks out of the junkyard! But I don't want to get overly obsessive (as I have done in the past) about it.

Good luck, Rg

[72CSTC5]

What everyone has said is true. We change door skins on new model cars, trucks, and suburbans that are dipped and galvanized coated and door skins are now glued, no longer welded and we will find rust down in the bottom of the doors when we peel the skins off. All you can do is the best you can, but the very best method is to have the vehicle acid dipped to remove all rust and then dipped again in e-coat primer just like a new automobile. Good luck.