Stripping factory primer
 

I have searched previous discussions and have found a lot of information about prepping for a repaint. My 1975 finish has a very crazed clearcoat finish on the horizontal surfaces and is nearly gone on a portion or the engine cover.

http://forums.pelicanparts.com/uploa...1065293532.jpg

The body is in fairly good condition and it is likely that little prior body work has been done

An Porshe experianced auto body shop owner advised me not to strip to bare metal because the factory "E-coat" is very good and is nearly impossible to duplicate. He advised I sand down to this green coat to remove the deteriorated paint.

He discouraged chemical stripping of the top layers only because that would weaken the base coat.

What do you all think?
Is the factory base coat worth saving in a car with good panels? Is sanding too much of a pain?
Do strippers cause problems with their residue?

So what are you saying, re-clear the facory base coat that is on the car? If it is a single paint job car and the base is in good shape, then scuff the entire down to the base removing the clear and then prime/seal/paint. The bottom line is the new paint needs a good substrate to adhere to.

You really don't need to seal anything except where body filler has been applied, just to clear that up. Also, the factory primer is extremely good. Any new primer you put on won't be as good.. The best way to strip the paint is to sand it. It will take you about a day to do if you have a good sander. Stripping isn't all that bad but you can really run into some big problems if your not very careful about cleaning and getting all the stipper residue out of all the cracks and corners. A big pain if you ask me... I usually just sand, especially if your going to use the old primer. After you sand it down to the primer, spray a good filler primer to fill the sanding scratches, then spray a guide coat, then block sand with 400 wet paper. After you get all the sanding scratches out you are ready to paint!!! Have fun.. The most time will be spent block sanding the car. Also, when you spray the filler primer on, spray a good 4 coats or so, as you will be sanding most of it back off when you block sand.
Feel free to sak more questions!!!

Also, one more option is to just respray. You just have to sand the clear off and then guide coat the base coat with 400 and then your ready for the topcoat...

Thanks for the responses!

Because the clear and possibly the base color is crazed (tiny cracks) I think it is best to remove the clear and color. I intend to do a full coat of the copper brown metallic color and then the clear - but the question is - do I start from metal or from the factory primer ("E-coat" is what I have heard it called)?

What method is best for sanding? What grit and what type of machine? Is a 7 inch sanding disc a bad choice?

You might find that the primer has nicks and scratches and maybe the crazing goes all the way down to the substrate. From the pictures you posted, I would sand the car down to the E coat only if it is in good condition. It looks pretty sun beat so the primer might be toast too. Be safe and strip it down with a DA sander, use a good prep coat, a GOOD urethane two stage primer then go with a base coat/clear coat system. Be prepared to spend about $1000 or more on materials.

Well, since the paint has cracks in it the rule of thumb for me has always been to strip to the metal. It will be hard to see the cracks in the primer because there isn't a gloss. The best way to sand a car is with a electric buffer with sanding discs. You can use 60 grit but be careful as you can actually sand into the metal if your not careful. Are you planning on painting it yourself??? What kind of tools do you have in the area of sanding? Also, do you have a spray gun if your planning on spraying? Also, one more thing. If you sand down to bare metal, you should plan on doing it in one long day, then the next day you can spray either a self etch primer on or an epoxy primer. These are the two options you have for the bare metal coat and which one you use depends on the time frame of your project.

Thanks again 80sc man.

As far a sanding equipment I have a long and short sanding block. I just bought a 7 inch variable speed 1000-3000RPM sander/polisher from Harbor Freight. I have a 3000 RPM random orbital polisher.

http://forums.pelicanparts.com/uploa...1065394684.jpg

I also picked up Harbor Freight's HVLP turbine paint gun kit - on sale for $70. I will see how it works on some juvenile furniture then perhaps primer. I don't have an air compressor so that limits my options - although I could borrow a compressor and gun if I successfully progress that far. I'm not dead set on doing this myself, but I would like to do as much as my schedule, the calendar (I like to drive this car) and quality allows. Luckily I have a painter who will work with me to finish the project if I stall out. I'd like to spend generously on the materials and use my own labor.

The paint isn't cracked per se. The clear may have the majority of the defects. This shot of the hood emblem shows the condition that I am trying to describe. Very fine pattern that likely does not penetrate to metal.

http://forums.pelicanparts.com/uploa...1065394863.jpg

I used 220 grit to sand the barest spot on the engine cover and soon got to an obvious layer of primer. The painter I talked to predicted that this was the oxidized base of the color coat - he was right - I thought it looked like primer or metal.

http://forums.pelicanparts.com/uploa...1065394978.jpg

I think that the plan of trying to sand to the primer would allow me to keep the "irreproducible " factory primer and avoid having to treat bare metal. It is likely I will be working on this project with unpredictable scraps of time so a single day strip to metal, treat and coat would be only a dream.

What do you think?

A couple of observations based on my recent experience:

Taking it down to bare metal is the only way to know for a fact that there isn't any surface rust. I found a half-dozen pin holes under perfectly good paint after having my entire car media blasted.

Be very careful with strippers. Any left on the car will "strip" the new paint as well.

There are products available that are as good, or better, than the original prep material. It boils down to how much money you want to invest. I thought I would die when I heard how much catalyzed primer costs. Then you follow up with a good sealer like DP 90, then your color coat... It adds up quickly.

Rich

Well..The best way to go about the project is to scrap all the old paint and start over. That way you know what your working with. But, if you want to just take it down to the factory primer that is also a good option, just not the best. It will save you alot of time and money though, and the results will really not be that different.

http://www.autobodystore.com/cgi-bin/config.pl?index

This is a link to a great board where they are all very experienced painters/ autobody guys and they can answer all your questions in a very clear way. Feel free to ask me questions too!!! I am on that board as well a fair bit. One thing to remember with painting, the prep work is everything.. and I really mean everything. The paint can only look as good as the prep work allows it. So if your doing all the p0rep work, ask alot of questions, and when you think your about finished with the prep, your probably about half wqay done!!!

Another question I have has to do with block sanding the 911 form in general. Using the 220 grit to block sand down to the primer I find that block sanding seems sort of ill-suited to the curves of the 911. There are areas that are fairly flat such as the doors, roof and hoods but the fenders are very rounded. My stripping seems to end up fairly smooth with the block on a fender but as I look ahead to guide-coat block sanding and finish sanding my clearcoat I wonder if there is a certain size of block and/or technique that is best. I have read that a longer block is best initially when taking down high-build primer and filler to improve shape. Than swithcing to a shorter block.

Any advise on blocking technique? Should this be another thread?

Thanks!

Well, there are all different kinds of block sanding.
When you are sanding the clear after your finished coat is on the car most people would say not to block sand, but to just sand with your hand, unless you have heavy orange peel or dust nibs.

1. When sanding out the scratches with a guide coat. This is done after your finished stripping whatever your going to strip, and then you have sprayed your 2K filler primer on. First you spray the filler primer on (4 or 5 coats) and then you mist a guide coat so that when your sanding you can tell you have gone deep enough when the guide coat disapears. When your at this stage, you will be wetsanding with a 400 grit paper. After this you should be ready to spray the topcoats on. Use a small block for this. If you use a big block it is really easy to miss a good amount of scratches.

2. When forming filler. This is really the only time to use a big block file. The bigger the area, the bigger file you will need. This does not really appy to you though because the body on your car is very straight right?

I spent several 10 min sessions sanding tonight. The oxidized areas of color came off very easily and the primer seemed solid. What should I look for to tell if the base primer is compromised?

I found that by putting a self adhesive 8 inch 160 grit pad on my orbital buffer that flat areas were tackled speedily but that I couldn't control the sanding over the fender curve and went to metal. There seemed to be a darker layer before the bright metal - what does this strata represent? a coating or metal/primer interface which has darkened?

I found that using a 220 grit block allowed me to move along the convex curve and control what I was taking off better. This gives me greater confidence in smoothing my 911's curves to perfection later. Good practice at this stage. I need to try the sander/polisher to see if that gives me better control.

As far as the areas where I have hit bare metal, can I sand oxidation off these in prepping before primer later? I hope to avoid these as my technique improves. I don't fear rust - it is currently VERY dry and still warm here.

As far as the body goes, behind the right rear wheel the fender got pulled out (before I claimed ownership) - I hammer/dollied this into respectable shape when I had my oil tank out. This and a few other areas are the only ones where a skim coat of filler may be neccesary above the multiple coats of high build primer to be guide coated down.

Thanks for your interest and advice!

Sounds like your having fun. Hopfully it will last!!! The dark layer was the zinc coating that makes the metal galvanized. Don't worry about taking that off, it's very over rated. You should spray something on the spots that went down to metal though, even if you just sand it off again later, it is not a good thing to have to worry about rust problems down the road.
Also, as far as the block sanding goes, your really just trying to get all the deep scratches out of the primer, not nessessarily trying to shape the surface into a flat surface. Hopfully that makes sense.]

Keep the questions coming!!!

If blocking is for scratches, which step is the most critical for creating a ripple free surface? There are slight undulations from dents removed and minimal door dings etc. Does the primer spraying step fill the low spots more than the high spots, that is Before it is block sanded?

Now that I have a stripping plan established, my next step is to start dismantling bumpers, and removing hardware before I resume striping.

Yeah, your right. There are different steps in the blocking process. The first step of the blocking process is to do all the bodywork, and you use a guide ocat and block sand. After the surface is perfectly flat, then you block sand all the scratches out. That is when you use the 400 grit paper and wet sand. You really should check out that websight. There is a video that the guy who runs the site sells., his name is Len, a really knowledgable guy.