Painting preparation: keep the filler or sandblast it off?
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    Default Painting preparation: keep the filler or sandblast it off?

    I've decided to start redoing the paint of some of my 1960s FP2 parts. The first piece is the long-reach head, which I originally took apart to clean and grease.

    The paint is a 2-component polyurethane (MIPA PU 200-90). I've had it color-matched, the correct combination was RAL 7010 (Zeltgrau) with 60 grams of white added per kg.

    Here is the first part I am going to paint, I've picked it because it's not very challenging, and if I screw it up, it's not going to be hard to redo. My question is, how to prep the surface?

    I haven't made any attempts to do high-quality painting for forty years, since my RC model airplane days (and the materials there were typically ABS plastic, fiberglass, and wood, not cast iron). A more-experienced friend has told me that the only way to go is to sandblast the entire part free of all old paint, spray some layers of filler, sand that and then paint on top. (He says that this is also necessary because oil and cutting fluid soak into the parts, and without sandblasting everything away, the new paint won't adhere).

    To me, this seems like a waste, since most of the paint and filler are intact. So I don't understand why I should remove these just to redo it. To see if this will work, I've wet sanded with 220 wet-or-dry paper, got off almost all of the obnoxious green stuff, leaving a nice scratch pattern for the new paint to adhere to.



    There is no bare metal here, just lighter paint under the darker paint, then white filler under that. The parts that appear shiny actually have a good scratch pattern, but the light reflection makes it look otherwise.



    (Note how the paint color has darkened over the years, from what it was under the nametag!)

    Next step is to use some auto-body filler to fill twenty or so small dents and nicks (they are typically a few mm wide and deep) sand again, and then paint.

    Is this approach reasonable? Or is my friend right, it's better to sand-blast off all the old paint and start over? (If my approach doesn't work well, I can always try his!)

    Cheers,
    Bruce

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    Strip and start fom scratch may be the way to go for cars or something, but seems like a waste of time here.

    A lot of the nicks would be removed by sanding the orig. paint a bit further, maybe a coat of primer then paint.

    The long reach head is awesome, it makes the FP2 so much more useful since the Y travel is so limited.

    Chris

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    The basic philosophy is that if the base material is stuck well enough that it doesn't peel off then it's a adequate base for your new coat of paint, no sense taking it off.

    As your new paint is a two part system it will work well as a sealer over the old material. I would apply a coat, using it as the primer. Let it dry, address any final repairs then apply your finish coat.

    Stuart

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    Chris, thanks for the note. This makes sense to me, apart from one point. The paint/filler film is only a fraction of a mm thick, whereas the dents are in places where the head was dinged and are this thickness or deeper. So I can't sand those out. And yes, the long-reach head makes a lot of sense, the high-speed head also has a substantial amount of extra travel.

    Stuart, thanks also for the reply. Is your suggestion to paint immediately, and only then to use body filler to fill in any dings? Then do a final paint coat? That certainly would make it easier to identify the dings, and if I use a grey filler, would provide a pretty uniform color base for the topcoat.

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    This is a tough one to answer. In the automotive finishing arena you would prep the surface, seal with a 2K primer, do any any filling and re-prime and seal with that same 2K primer then scuff and finish with your final color.

    This seems like a lot of hoops to jump through for a machine tool so maybe you could modify it a bit. You're going to have to make sure the reduced paint you're going to spray over your part doesn't lift the existing paint.

    I would do all the Bondo work first then spray a coat to seal the whole thing. Do a final scuff or light sand and finish coat the part.

    Stuart

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    Complex question here...
    If the original finish is still well bonded to the base casting, my take would be to leave it alone.....My personal belief is that the finishing industry is an area that science and technology have
    not made better over older traditional materials.....
    Environmental and health restrictions have limited the development of finishes (consumer grade) to the point where they are just not as good as they used to be.

    I would not use Bondo for filling small scratches and chips. Bondo in general is relatively porous , plus the mixing and all makes it less desireable for lots of small work.
    At your local auto body repair supply shop you will be able to purchase "spot filler" which is usually a lacquer based high solids filler in a tube that you apply to small spots.
    Once it dries it can be sanded like primer, is easier to work so you won't sand low spots around the perimeter (as with harder bondo)...

    I would only go with Bondo where you need to repair the profile of a section of the part....The epoxy, metallic bondo's are very good, but harder to work.There are also spray on fillers for
    surfacing of areas where you want to improve the surface smoothness.

    Don't mix products....use the same makers primer and finish. I would encourage you to investigate primers that contain zinc and "etch" the surface they are applied to. This will give better bonding
    to the original surface coating and help with grip on any bare cast sections. Etching primers are generally followed by a filling primer, then color.

    Be careful with the Urethane finishes!!!! Highly toxic, should have a suit that covers all bare skin, and an air mask to supply outside air for breathing.....
    Also the Urethane finish IMHO while are very tough and chemical resistant and will give that slick wet look, lac chip resistance that some of the less rigid finishes have.

    Cheers Ross

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    Filler adheres the best to bare metal So I always take a diegrinder for small spots and a angle grinder for bigger spots and grind to bare metal Then apply the filler
    If i realy want to do a good job I then apply poly That is a sprayable polyester filler It is about as thick as custard and you can get a dry layer of about 1mm It sands real easy too But you cannot apply it on all paints

    Another good advice it to sand the corners to bare metal at a 45degree angle Thats the place the filler chips easy otherwise

    I apply the paint with a roller not the foam ones but the fabric ones with a smaller diam I get good results after I sand with grit 150 or so
    spraypaint needs a 1000 grit wet sandpaper or so


    Peter from holland

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    Yes, Peter, I do agree. Sprayed polyester filler is great for the machine, but bad for the shop. At best it is done outdoors or in a spraying cabin. It stinks polyester to high heaven! The roller approach (for topcoats), on the other hand, is much more environmently friendly. With a roller you may even use polyurethane without the risk of inhaling particles. High gloss is to be avoided, it turns you into a neurotic. 70% gloss is great.

    Ole

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    Dear Peter, Dear Ole,

    Thanks for the hints and suggestions. I've ordered a couple of more small containers of filler and putty, and will do some more work on the head this weekend. I'll post a report once I have figured out what works well for me and what doesn't.

    One bit of good news is that I have confirmed that my HVLP sprayer (normally used for water-based woodworking finishes) can spray 2-component finishes with a viscosity of up to 170 sec (DIN 4). The MIPA finish I am using is 130 sec undiluted, so should work fine.

    Cheers,
    Bruce

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    cleaning a sprayer used with water based paint is easy, cheap and non-toxic
    two components is another ball game.

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    Quote Originally Posted by ole.steen View Post
    High gloss is to be avoided, it turns you into a neurotic. 70% gloss is great.

    Ole
    I always use high gloss Applied with a roller it works great
    High gloss is a stronger paint and it will turn pale in time anyhow


    Peter from holland

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    I think that preparation is everything and having tried all different methods I do this, pressure wash the parts with aggressive detergent and then do the rubbing down with waterproof paper and water, I coat the paper with hard soap and this removes remaining traces of grease.
    I then fill with 2 part body filler which is easy sand, I do,not know the US equivalent and rub again.
    While spraying with modern paints is the 'new' way, remember that the factory used cellulose both as filler and finish and this has lasted well, our finish must adhere to that.
    My last rubbing down is with about 240 grit wet paper, I then mask up and apply a couple of coats of thickish red primer by brush, this is designed to stick to any thing and to let any paint stick to it.
    I the rub with 240'again instil the primer has almost disappeared, this shows the defects up and so fill and paint again until the finish is OK.
    The final finish is up to you, I use top quality brushing lacquer, rubbing down with soap again using very fine paper 500 to 1000 grit.
    After allowing to harden you could use burnishing paste but I think this is going too far.
    One advantage of the brushed finish is that you can regulate the thickness of the paint and touch up the small marks that you will leave on assembly.
    I have machines in my shop done 30 years ago which still look good.
    I have found that water based paint is poor at adhesion and is not durable even to the extent of peeling off.
    Peter

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    Quote Originally Posted by peterve View Post
    I always use high gloss Applied with a roller it works great
    Peter I think I might also experiment with a roller. Can you please recommend one that works well (specific brand and model) and which I can buy in Germany? Cheers, Bruce

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    I've not had much time to work on this, but am making some progress. After Ross' warning about the dangers of 2-component paints, I did some research. In Germany, you can work with these paints for periods of hours with a properly fitted A2P3 mask, so I got one from 3M. I also made a painting area from some cardboard glued together with hot glue, and fitted with an exhaust fan in the window, and experimented with my HVLP gun. The fan is a 20" box fan and moves enough air that when I am wearing the mask and painting, I feel quite safe.

    (In principle the box fan moves 1500 CFM, and the cross-section of the opening of my painting area is about 10 square feet, so the air velocity there is about 2.5 feet/second, roughly one meter/second.)

    Here is after a couple of rounds of priming and filling.



    Note how little overspray is present: the HVLP really puts the paint on the target and not in the air.

    The head itself and sheet metal guard I sandblasted clean of all paint and filler and did from bare metal. The long arm I sanded down to filler and then primed on top of that. My conclusion from this comparison is that it's better to use the existing filler, not to sandblast all of that existing filler away.

    The new filler/primer coat is not perfect and needed a little bit of sanding. I think if I adjust the HVLP gun for a somewhat wetter topcoat, I can get that right the first time and some rubbing compound and wax will be all that is needed.

    The 2 component paints I am using (from MIPA) have a longer pot-life than I had expected, at least in the temperatures I am working at (around 10C, the minimum for these paints). I can easily get 4 coats, waiting 15 minutes between for them for the solvents to evaporate and the paint has not started to harden in the pot.

    Cheers,
    Bruce
    Last edited by ballen; 03-16-2016 at 06:16 PM.

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    I'm back after some travels and have done the first top-coats. Here's the arm:



    and here is the head and Z-axis shield, with the painted parts on top of a warm radiator to help speed curing



    It's been a while (40 years?) since I did any real spray painting, and on much of the area I was too worried about runs and didn't spray enough paint to smooth out completely. Hence there is some slight orange peel and texture.

    So to get a really good finish I'll have to wet sand and rub this out. Does that work well with two-component PU paints? Is 2000 grit (wet!) abouut right?.

    Cheers,
    Bruce

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    Looking very nice Bruce.

    Don't sand / grit blast flat panels, they'll warp all over the place. I made that mistake on some skirt panels on my Fanuc Wire EDM - never could get them flat again and ended up making new ones !

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    Hi Andrew,

    I sandblasted the sheet metal cover, and the head. But the sandblaster that I use is a small hand-held one, with a working area which is about 1cm x 1cm. So there is no danger of damaging or distorting anything thicker than aluminium foil.

    Cheers,
    Bruce

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    Quote Originally Posted by ballen View Post




    So to get a really good finish I'll have to wet sand and rub this out. Does that work well with two-component PU paints? Is 2000 grit (wet!) abouut right?.

    Cheers,
    Bruce
    I would check if the paint in the runners is hard enough to sand before doing anything
    I would start with 800 and then 1000 1500 and then 2000. I have 2000 but it might be hard to find. With wet sandpaper grit 1000 you just needs some more buffing


    Peter from Holland

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    Looks very labor intensive but if its what you love all the more power to you.

    If it was me I would sandblast and sent out for powder coat in polyester, tougher than any paint and will last forever.

    Epoxy would be my second choice.

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    It's worth using a guide coat to guide your sanding. After the two part paint has cured, get a tin of spray paint (cheap automotive gloss black). sputter the paint over the surface to leave a very thin layer of paint droplets all over the surface. Let that paint dry and the use the 1500 or 2000 grit wet and dry paper to sand the surface. When you have removed all of the guide coat, you have got an even surface. It's really a waste of time, but it makes you feel good.


    When you've gone as far as you can, you can use polishing compounds like these. The finish won't last once you get chips and tools dropping on it. But it makes you feel good!



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