Sanding a failed clear coat.

[ms fowler]

YES, I know, this is Garage Forum; not Paint and Bodyshop Forum. But my observation is that usually several someones can give reasonable answers to almost any question--don't prove me wrong....
I am sanding a failed Clear Coat--repaint about 6 years ago of a failed Clear coat on a '82 Mercedes 300SD. Two observations--1) the Clear Coats--both factory and repaint are HARD. 2) When sanding the repaint Clear coat, I smell the solvents. Is that normal? What, if anything, does it mean. No odor form the factory Clear.

Anyone have any experience with electric DA sanders? Its seems wasteful to run a 5HP electric motor to run the Air Compressor to run the air-powered DA. A straight electric DA just HAS to be more efficient. Are they any good? I need 6".

 

[johninct]

I have a 1983 300SD. We repainted it back in 1992 with Mercedes factory package paint and it still looks great. I am not sure what you are trying to do. Scuff the clear and just re-clear? Sand off all of the clear then new base coat/ clear coat?

 

[ford33]

It will be difficult to remove just the clear coat layer. Unless you are very careful, you will sand into the base coat and then into the primer. It is better to remove the clear and base coat and just repaint the entire panel. You cannot touch-up just the areas that the clear coat failed. Colors and sheen will not match.

 

[tcianci]

It will be difficult to remove just the clear coat layer. Unless you are very careful, you will sand into the base coat and then into the primer. It is better to remove the clear and base coat and just repaint the entire panel. You cannot touch-up just the areas that the clear coat failed. Colors and sheen will not match.

This! Another thing to remember is that the clear that you are blending into is OK today but tomorrow it's the start of your next failure. Take the panel down to the primer, seal and re-base/clear

 

[padroo]

I suppose you could use an electric random orbit DA sander especially if you only plan on painting one car but it has to be random orbit.

 

[fastjohnny]

Sounds like the car needs to be stripped...a paint job is only as good as the substrate below it. Which has already failed. Twice.

You are not smelling solvent, that evaporated long ago. You are smelling the paint solids.

 

[ronnyg801]

The solvent can still definitely be trapped. if materials are applied too heavy and not enough flash time in between, they can be trapped. sanding into it, if you are smelling actual solvent, this is most likely what has happened. it is also possible you are smelling dried plastic IE paint. From my experience properly applied and cured paint, holds little to no smell when sanded. Hence why the factory paint doesnt have a scent. it is also very likely that the factory paint was helping hold on to the thinners/ reducers/ dilutants of the second paint job, again long enough that it was trapped.

Regardless of that.

You want to remove off any material that is failing. But also be aware that automotive paint is only designed to have a mil build of around 12 mils. Older cars came out of the factory with possibly 5-7 mils. More modern stuff, we are seeing 3.5-5 mils in some cases (cost cutting more less) so, that leads the common mindset to allow around 3 paint jobs before the panel or car needs to be stripped. Or at least enough material is removed that once more is applied you are under that mil build. good luck doing that... it is more common to strip to bare substrate.

depending on the electric sander and the air powered you may be getting different performance out of each as far as effectiveness on how fast they sand. In my opinion that cost of running the machine on one job should remain irrelevant, over being able to work faster and being able to get the job done faster in efficiency. If costs you pay to the electric company are truly a concern you can certainly sand it all by hand

But you are correct in that if you have two sanders that are apple to apples in what they do, an electric one is going to be more efficient on electricity to running a compressor, ha ha

The mentioning above about your new paint is only as good as whats under it is absolutely correct. You wouldn't want to build a mansion on a swamp... Same mindset here. the material used from the factory in '80 is far inferior to what is the standard of today.

If you truly want something to last forever, taking it down to mare metal isn't a bad idea, as long as you are building it back up with high quality materials designed to last. chances of something cheap being better than what was at the factory in 80 is very good. But if you are taking all the time to go down to bare metal, might as well spend 20% more money and used good stuff the whole way through.

If you just want it to look good for the next 5 to 10 years, sand down the failing areas, prepare the rest for paint and put what your budget can afford over the top, of automotive refinish quality. If you really want to take it down to bare metal and build it back up, it is going to be a long haul, take the time you think it will take and probably triple it. Its like a remodel or renovation on a home etc. You will find things you weren't anticipating, then you get to figure out what effort you are willing to put in to fix it.

I am not sure any of this really helps, as I am not sure what the real questions were.

I am a paint geek by trade though and have worked in the collision repair refinish industry for the last 13 years . This place shares so much knowledge for me, I try to chime in when I can with something I know when I get the opportunity.

 

[ms fowler]

Thanks for not disappointing--I KNEW there was expertise here--on almost any subject!

This will be an all-over repaint; not just a few panels. I plan to respray with a SS urethane--if there is no clear coat, there will be no clear coat failure!
I didn't really want to go to bare metal as the factory process was pretty good, but the substrates under the clear are soft by comparison. When I am sanding, the time from when I stop seeing white residue ( clear) until I am into bare metal is very brief--like seconds. I am using 60 grit to remove the Clear ( and almost everything else). I will follow with progressively finer grits and then begin to build back up with a primer all over. It will act as a sealer for any remaining finishes.
Yes, it is a long process. I understand that the prep is the most important part of any paint job.
Thanks again.

 

[PWC Repair]

I use an electric DA for bodywork/paint. They stock 5" (common for electric) and 6" paper at my local bodyshop supply. Clearcoat doesn't just fail for no reason. 5-6 year failure is always from cheap clearcoat. Use a quality clear and wax it regularly and it will last for years upon years. Try to at least get through most of the white clear then switch out to 180 grit and go over everything into the base, then 220-320 grit over everything again. Then you'll be ready to start the various primers and paint.