Any guidance on the door sill rust

2015 Subaru Forester, ~45K on the odometer

While cleaning I noticed very little rust near the door sills so removed the protector. I noticed a good bit of surface rust beginning to build.

View attachment 27595

YouTube videos suggest to sand it down to the metal, primer application, few coats of the paint and then the clear coat. Is that the best method?

If yes, what grit sandpaper should I start with?

Once it is fixed, does it make sense to put protection film in that area?

Any other guidance is appreciated.

Thanks in advance.

I have been thinking about this one since you posted it, there are lots of ways to repair this but many of them require a lot of work, spray guns and other things along with the skills. Not really DIY friendly.
I would get a few things together before I started like newspaper, vinegar, 400 and 600 grit wet and dry, a rubber sanding block, a dremel with sanding attachments, self etching primer, regular light grey primer and paint.

Start by sanding the rusted area down with a dremel (a small drum used on an angle would probably work best), do not spread out the sanding too far try to keep it as localized as possible.
Using the block with 400 just use the block on an angle to sand the area down about an inch all around from and including the rusted area. Clean it with water and wipe dry.

Take some newspaper and make a small roll about the same size as the area that is now naked sheet metal. Soak it in plain white vinegar, squeeze the excess out and put it on the rusted area for a few hours. This is very effective and cheap and will remove any traces of the rust.

Mix a little baking soda in warm water and wash the area, rinse with water only and wipe it thoroughly dry.
Mask off the area leaving just about and inch around the clean metal, turn up the edges of the making tape around the area, this will prevent a hard line. Put 3 coats of the self etching primer (rattle can), let it dry between coats then spray the regular light grey on top of it for 2 coats. Unmask it.

With water and 600 grit sand the primer smooth, try to be gentle so you don't break through, you can touch it up if you do.
Wipe it dry then mask off the area using the same turned up taping method and spray the paint, 3 light coats or until covered.
You can use base and clear from rattle cans in the correct color or have an autobody paint supply shop mix you a single stage.

I would put a layer of 3M film over that area once its totally dried. Make sure to round the corners to prevent lift up.
PM if I can be of help or you need to clarify the instructions.
 
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Cujet, any increase in corrosion resistance is only minimal with HSLA steel. It is not a consideration when corrosion resistance is the main factor. Even your link states HSLA steel rusts.

Can you elaborate on how a choice of steel can factor in with rusting, and what was wrong with the steel Chrysler used that gave them a problem? Usually the problems are with the coatings, and not the steel itself. What was intrinsically wrong with the recycled steel that made it prone to corrosion?

Kestas, you may find this interesting. I know some Euro cars are using a higher grade sheet metal on some better models. The sheet metal I get from a fabrication shop to do patches is very corrosion resistant without any coating. It does get some surface rust after a while but it seems to stop there, I assume it has higher nickel content.

 
Its not cheap but look at a Mazda if you want to see cheap sheet metal then look at a 20 yr old Euro car (not all but many use a steel alloy), they may have some rust but nothing like a Mazda.
Get a paint chip on the rear quarter on a Mazda and its like lighting flash paper.
 
I keep reading where people talk about "cheap steel" as the reason for rusting on cars. I wish people would stop with these posts. It's not cheap steel, per se, but the corrosion protection that is applied to the steel, which determines the corrosivity of automotive panels.
 
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I keep reading where people talk about "cheap steel" as the reason for rusting on cars. I wish people would stop with these posts. It's not cheap steel, per se, but the corrosion protection that is applied to the steel, which determines the corrosivity of automotive panels.

Sorry but I respectfully disagree with you. Please explain to me why a 1999 Volvo has paint missing down to the sheet metal on the rocker panel and surface rust but still has not gone through in 5 years, yet a Mazda with a small paint nick has 10" of the dog leg gone in a year despite being hot galvi dipped.
It has nothing to do with any coating it is the quality of the steel used.

I have been doing car bodies for over 40 years and I can tell you there is a difference in the steel used not just the thickness.
Like I said I buy panel steel from a fabricator 18ga and the difference between that steel and the 18ga stuff from a car parts store is night and day, neither are coated in any way (welding through some coatings can cause some really bad fumes).
 
Have you ever tried to find out why there were differences? Have you looked into the chemical an metallurgical differences between the steels?
 
Yes, manufacturers use both hot rolled, cold rolled and different grades. Some use low grade sheet metal while others use sheet which has a slightly higher nickel content and has much greater corrosion resistance and longevity before coating.
As with sheet aluminum sheet metal comes in more grades. Vehicles using higher grades are becoming less and less, it is hard to put the amount of content today's consumer demands at an affordable price point, something has to get the short straw that the consumer cant see.

Look at older Volvo and Saab, I use these two because they were built close to each other and used most of the same suppliers for metal and parts. They used high grade Swedish sheet metal and a lot of higher quality components but compared to some lower priced US domestics they had less feel good toys, they were mostly all business.
Much of price of the car was in its structure.

We see the same thing when it comes to frames, some are still going strong at over 50 years old while others are rotted out at 6 years in the same climate and area
 
Is that from the door seals rubbing the rocker panel, or is that rust buildup/stains from the door rusting out? The type of seals on these cars is a death sentence in the salt belt. First thing I did when I got my Forester home from the dealer was pull the door seals off and throw them in the spare tire well. My truck is missing the bottom inch of the passenger door from this type of seal holding moisture in the door. They also rub the paint off the rocker panel and can lead to this.
 
It clearly looks like corrosion from friction chafing the paint. It is due to abrasion. It is not rust through from behind else it would not be in a straight line like that.
 
Mazdas are well-known rust buckets. Not even sure why this would become a topic of debate.
 
I have been thinking about this one since you posted it, there are lots of ways to repair this but many of them require a lot of work, spray guns and other things along with the skills. Not really DIY friendly.
I would get a few things together before I started like newspaper, vinegar, 400 and 600 grit wet and dry, a rubber sanding block, a dremel with sanding attachments, self etching primer, regular light grey primer and paint.

Start by sanding the rusted area down with a dremel (a small drum used on an angle would probably work best), do not spread out the sanding too far try to keep it as localized as possible.
Using the block with 400 just use the block on an angle to sand the area down about an inch all around from and including the rusted area. Clean it with water and wipe dry.

Take some newspaper and make a small roll about the same size as the area that is now naked sheet metal. Soak it in plain white vinegar, squeeze the excess out and put it on the rusted area for a few hours. This is very effective and cheap and will remove any traces of the rust.

Mix a little baking soda in warm water and wash the area, rinse with water only and wipe it thoroughly dry.
Mask off the area leaving just about and inch around the clean metal, turn up the edges of the making tape around the area, this will prevent a hard line. Put 3 coats of the self etching primer (rattle can), let it dry between coats then spray the regular light grey on top of it for 2 coats. Unmask it.

With water and 600 grit sand the primer smooth, try to be gentle so you don't break through, you can touch it up if you do.
Wipe it dry then mask off the area using the same turned up taping method and spray the paint, 3 light coats or until covered.
You can use base and clear from rattle cans in the correct color or have an autobody paint supply shop mix you a single stage.

I would put a layer of 3M film over that area once its totally dried. Make sure to round the corners to prevent lift up.
PM if I can be of help or you need to clarify the instructions.
Thank you very much. I was hoping you would comment because while others have given great suggestions, I was very confused about the steps. You made my life extremely easy. I will tackle this on a weekend, hopefully.
Thank you again.
 
Is that from the door seals rubbing the rocker panel, or is that rust buildup/stains from the door rusting out? The type of seals on these cars is a death sentence in the salt belt. First thing I did when I got my Forester home from the dealer was pull the door seals off and throw them in the spare tire well. My truck is missing the bottom inch of the passenger door from this type of seal holding moisture in the door. They also rub the paint off the rocker panel and can lead to this.
I will appreciate it if you could post a picture of how it looks like after those sill protectors are removed. The reason, those 'sill protectors' are there to actually cover, what looks to me like welds of the body and the door (sills). If I remove that protector, it will be no time, the rust will spread to the body.
 
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