Sunroof Roof Rust - Paint & Body Help Needed

[02coastie]

Quick background: My '74 coupe was originally my father's, who had it completely stripped and properly painted back in the late 80's. In addition, it was waxoyled (or similar) by the original owner. Combined the coupe is astonishingly rust free, particularly in all the usual spaces (rockers, firewall, fenders, etc). When I purchased the car back after a 20 year absence (see:A Father's Day Tale ) all the horizontal surfaces were experiencing clear coat failure to some degree as the car had been kept outdoors and uncovered.

I now have some limited funds to protect the car and give it an "above the belt line" repaint. Since the horizontal surfaces are in great condition, this seemed a reasonable approach. Prior to prep, I was aware of some small bubbling to the rear of the sunroof opening. Apparently this is a known risk area. I just came back from the paint shop where I found more significant damage than I expected. I wasn't even aware of the forward section. Only one area appeared to have been filled/sanded during the previous respray. The rest have grown since then. The question is: What now?



While the body shop is competent, I'm not sure cutting and metal replacement is best left to them. They have zero experience with coupes (good old local FL boys...mostly domestic work). If I had the budget and time, I would love to send it off to a professional, BMW knowledgable shop, but that's just not in the cards right now. My goal is to do the best repair I can to get the most time and enjoyment out of the car. It will always be garaged with limited/no water exposure.

They are recommending using a product called "tiger hair" which is basically a fiberglass resin to fill and seal, after doing some cropping and treating with Ospho. The shop owner seems to think this is the best way forward and should give me 10-15 years at least without any further visible deterioration. Does anyone have any thoughts on that approach or alternate recommendations?

Thanks!
-Lloyd

 

[Markos]

Hi @02coastie,

Take a look at the build thread by @JetDexter. He covered this in his build thread. I wouldn't go with "tiger hair" without addressing the underside of the roof, as highlighted in his thread.

 

[stphers]

Only way to do it is to cut out the damaged sections and weld in new metal, then treat it all in at least 18" in every direction after welding The " tiger hair" is just reinforced bondo with fibreglass strands running through out it It will all come back within a year or so Metal is metal If they are capable of doing high end jobs on American metal , it's no different to weld and metal finish

 

[02coastie]

Thanks @Markos for the link. That is the most detailed look at the problem I've ever seen. While I will obviously have to follow @JetDexter in the future, that level of scope creep is not within this shop's capabilities or more importantly my budget right now.

@stphers Understood that is the "right" way.

Other than having to do it again in the future, are there any other concerns with the stop gap approach as indicated? Anything else I can do to help prolong cropping out the entire section, other than parking it indoors/limited weather exposure? I'll also make sure the drains are free and clear.

 

[JetDexter]

I am not the expert here but I thought I would at least mention this: My car did not look much worse than yours from the top, yet it was completely rotted out underneath as you can see from the photos.

I understand that this shop isn't up to doing this the proper way, but they could at least drop the sunroof track so you can get a look underneath. Then if you can't replace the metal, at least they can sand off the rot before attempting a repair.

Equally important is the sunroof frame itself will be rusted where it meets the roof. Mine was toast but perhaps yours could just be sanded and coated.

All this to say that doing a fiberglass type repair is not ideal by any stretch, but at least drop that frame down do whatever you do can be done after seeing and evaluating the rot below. Then perhaps the fiberglass trick could give you the few years of enjoyment you are wanting on a budget.

While I would never live without a sunroof, it might be worth considering filling this in. Any decent shop should be able to do that, and the rust would never come back

 

[Honolulu]

Lloyd

I have some familiarity with fiberglass, having made surfboards for decades, so I took out the sunroof panel in my E30, cleaned off the rust I could, used Ospho, and fiberglassed the holes. It looked great for a while but rust simply came right back in six months, from under the patch. Removing/reinstalling a sunroof panel is a new skill and not like plain old mechanical work. Not difficult, do-able, but needs patience, understanding and clean hands so you don't muck up the interior trim panel.

Looking back, a better product than fiberglass needs to be used to seal the prepared metal surface. Perhaps some POR-15 paint, perhaps zinc-rich primer. Perhaps (I hope) others will chime in on materials and techniques.

I dunno the level of ability of a "decent shop" referenced above, but I thought I had a good shot at it, and turned out to be wrong. That shouldn't stop another person from trying, just don't use my method.