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So I fixed the rust spot under the door, but that just made the one on the fender look much worse!

So, now I have the fenders off to patch in some steel... before I put them back on I was thinking I should do some undercoating, sound-deadening, or preferably both.

I expect to weigh the little car down with a few rolls of Dynamat in the passenger compartment but I'm not sure there is any real benefit to doing the outer or inner fenders, has anybody got concrete experience in this regard?  I'm pretty tempted to just hit the area with POR-15 and hose it down with truck-bed liner in nobody has a better plan to reduce the noise and prevent rust moving forward.

I know the treatment could double the weight of the fender, but I'm getting soft in my old age and want to retain what of my hearing I can.

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Take the door cards off and dyna mat the inside of the outer skin too. Go up under the dash as far as you can. There is probably a thin board behind the rear seats so cover i. Lift the rear seat and do the floor under it. What's 20-30 extra pounds? compared to the bliss of hearing you stereo? and being able to hear people talk without raising their voice out on the highway? 





18 minutes ago, Mattndew76 said:

under coating is a good way to rust your panels back up.  Stick to sound materials and believe it or not but price does make a difference.  Look into D shaped weather seals in the door gaps.





Matt I agree, I wouldn't use that shit either but this is misleading. Someone covered the rusted out shit to hide it. If that was on there for long enough to cause that it wouldn't be so black and fresh. It would work much better if put on new to prevent rock chips taking the original dip off the frame and suspension parts. What a nightmare to work on that!

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Indeed, seals are critical.  Door skins, firewall, and roof are the most effective places to dynamat for sure and I'll eventually be doing those areas and many more.  I just have to address the fenders and inner fenders right now since I have things apart already.

It is also the case that undercoat on top of rust damage will accelerate that rust damage.  It is interesting to read the P-sheet for PPG RoadGuard noting how many products it is specifically NOT OK to use it on top of: http://secure.hmswarehouse.com/TECH/DELTRON/DX54.pdf

That said undercoat applied to properly prepared clean metal has been a critical part of a corrosion prevention package that works for quite a long time and I'm not sure what, if anything, has replaced it. 

I can apply an epoxy base coat on the bare metal, but I have to spray something on top of that or it will eventually get beat up by rock chips and I'm back to patching panels.

The local DuPont paint supply shop suggested truck-bed liner (ideally "lizard Skin")

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If you are going to under coat the vehicle be sure to do it over prepped paint and not bare prepped metal. Even Por-15 is not resistant to all rock damage. Once its been compromised the water will get in and reside under the rubberized layer.



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 I was pretty impressed by the test video, reasonable test protocol and good things to consider for sure.  Unfortunately for me, he didn't do anything with POR-15, DTM epoxy primer, or other factory type coatings which I'd like to think will hold up better than simple add-ons. 


 Since it's a California car and the corrosion is mostly not too bad, I' now feeling like a walnut-hull or glass bead blast on the worst areas.  With DTM epoxy on the areas that come clean and on the stuff that won't reasonably clean up POR-15 and a tie-coat.  Then a couple coats of SS 2K urethane on all that and finally some sort of flexible material to take the hit from rocks and gravel and protect to Urethane.

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