Body work question

[slumlord44]

I'm patching the lower rear section of a '65 Mustang front fender. Cut out a 6x6 section of rusted metal and made a patch to weld in. The inner support is rusty but solid. Plan is to clean it as well as possible and prime and seal it. Suggestions for products to prime and paint the inside. Considering weld through primer and POR but wondering what others are using. I will be butt welding the patch in.

[Michael Allen]

If you can remove all the rust, most Automotive Paint Manufactures (PPG, Axalta, Sikkens, Sherwin Williams Automotive, NAPA Martin-Senour) Direct to Metal two component Epoxy Primer will do a great job; topcoat with the same company's two component single stage finish. Weld Thru primers work well but are usually used in places where painting is not possible after welding (such as a rocker panel). Self Etching primers usually require a really clean surface and require a topcoat. I prefer the two component epoxy/two component topcoat method. On the less costly method; RustOleum rusty metal primer plus a topcoat works quite well.

[Ford blue blood]

Clean all the rust off you can get at then apply a coat of rust converter. Ospho (sp?) is a common product. Let it cure for at least a full day then top coat to seal it from the elements. After welding is complete clean the area as well as you can and saturate with a product like Rustoleum.

[slumlord44]

Thats prety much what I was thinking.

[Daves55Sedan]

For portions of rusty metal brackets and panels to remain, I have always cleaned them down to bare metal using cheap wire-wheel attachments in an electric drill. When the wire-wheel becomes dull, just shift the drill in reverse and keep going. Repeat as needed. You can often get down to bare shiny metal.
In case of pitting, I always use acid etch on the bare metal. After all the welded patches are done, clean off slag with a wire brush and acid etch again. I suggest getting an empty tuna-fish can, bend a spout in the can and pour some acid etch in it. Then turn the fender somewhat upright allowing it to rest on a carpenters sawhorse. Then take the tuna can and pour acid etch down in between the structural bracket on the backside of the fender and your new patch panel. Allow to dry thoroughly. Use the same can to pour a quality red-oxide metal primer down into the same area. Possibly repeat for overcoat enamel. Of course a drop-cloth or newspaper on the floor will be needed. The exterior, just finish as you normally would a new metal surface with spray primer and enamel overcoat.

[john worden]

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I agree on the use of a rust converter. Don't cut corners after that though.
Prime with epoxy primer and paint with catalyzed paint.
Think about it. It rusted because it trapped moisture and road debris. The factory did a poor job of protecting the new metal. Do a better job now with the newer products.
John

[rotorwrench]

I've always used systems for aircraft painting but they are getting expensive. I still use zinc chromate primer but it's getting hard to get. Some folks have been having good luck with the Por15 stuff. I may try it on the hidden areas when fixing the crusty rusties in the old rockers & floors. They have their own phosphate etch but I've been using Ospho for years with good luck. A person should use a conversion coat like Ospho if it isn't practical to remove all the rust pitting. Iron phosphate is tough stuff. It doesn't build surface corrosion on it unless it still had oil or grease on the surface where it wouldn't work. Sometimes it takes a while to dry but a person can't rush that.