First Maintenance on New E30 - Trunk Leaks & Rear Inner Arch Rust

[c0rbin9]

I'm relatively new to working on cars and wanted to make a thread hoping to clarify a few things.

The car is mostly rust-free, but has some surface rust in the trunk due to water intrusion. The water gathers in the spare tire well.

My goal is to fix the leaking and sand/paint any areas of rust. Here are my questions:

1) I have a pint of leftover Brilliantrot paint that was used to respray the car a few years ago. Can I just use this paint for the rust spots? Just primer and then brush it on with a small paintbrush? I was thinking maybe I had to mix it with water or something... I've only used touch up paint and spray paint.

2) The surface rust in the trunk looks easy enough to sand and paint. However, I can tell that there is some corrosion directly forward of the battery box, in the "crevice," and also along the seam that makes up the rear wheel arch. My question is how difficult would this be to cut and weld... seems more difficult than the battery box. Is it made more accessible with the battery box cut away? I don't see how to get at it without cutting the quarter panel. Come to think of it, I can't even see or feel the area I'm talking about.

FWIW, there really isn't THAT much rust I'm talking about. The battery box definitely has some and so does part of the arch area, but it's not very extensive. That said, I don't want it to spread. Especially here in Houston where it's so humid that even glass rusts.

Gah, body work sucks.

[itsonlygeorge]

Pictures would help.

[Skarpa]

This is the process I use for surface rust:

- Remove all paint and loose rust from the area. Sand blasting works fine if you have access to equipment. I don't so I use grinder with steel wire wheel. Remember long sleeves, gloves and face/breath protection. Flying bits of steel wire can be painful.
- Apply phosphoric acid on surface to tackle remaining rust. You can buy this stuff with various names like rust eater etc. It removes rust and makes it chemically inert. Use according to instructions of the product. Usually you dilute it with water, spread it on surface, keep the surface wet for 20-40 minutes and then wash it away. Remaining rust turns black and the surface gets dark grey or yellowish colour.
- Now you need to prepare the surface for paint. It needs coarse surface to grip on. I use sanding pads or drill with nylon wheel. At this stage I also try to remove any spots of rust that the phosphoric acid made visible.
- Remove any dust with vacuum cleaner and wipe the area with grease/silicon remover. You can get this in spray cans.
- Now the area is ready to be primered. I'd advise you to use only two-component primers and paints. A layer of single component primer under two-component paint can cause the paint layer to bubble or peel. Your best bet is to check the manufacturer of the paint you have and use the primer they advice to use with that. Check if the paint is water dilutable or solvent dilutable. I can only instruct you with solvent dilutable paints because that's what I use myself. Normally you mix the paint with the correct amount of hardener and add some paint thinner and then stir it well. For touch up job you only need to mix a very small amount of paint. Brushing is ok but it will of course leave brush marks. Paint the area with a thin layer of primer and let it harden for at least 24 hours.
- Now it's time for the actual paint. Use the same procedure as with the primer. You can do a couple of layers of paint.

For the questions about cutting and welding: The pictures would indeed help a lot but if you're talking about rust in the actual wheel arch where the quarter panel meets the wheel well then I'm sorry to say that any hidden rust will soon be visible rust and it may be impossible to repair without cutting the rear quarter. It may not be the best location to start learning bodywork and welding.

[c0rbin9]

Thank you for the reply Skarpa, I'll update this thread as I do the repairs.

[c0rbin9]

Anyone know where that leak is coming from?

[c0rbin9]

bump

Seems like either the rear window or the rear quarter glass seal. Going to check those this weekend.

[KrautnotRice]

Tail light gaskets, trunk seal and antenna grommet are the usual suspects. Of course it could leak from elsewhere, but I'd start there.