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Old 26th May 2014, 06:07 PM   #1
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Default Block sanding trials and tribulations

Now that it is warm again, I am back to painting my Proteus and Mezzo Proteus speakers. I had to put them away for the winter because it was too cold to paint.

I'm painting them with rustoleum high gloss paint (from a rattle can) and sanding and polishing to a mirror finish. That is the goal at least. After numerous attempts to cheat (be lazy) by using an electric random orbit sander, I have resigned myself to the fact that there is just no shortcut to block sanding by hand. My sanding block is a rubber one made by 3M.

My process has been to spray two coats….allow to dry for a week…..sand until there is no orange peel…..clean the surface with mineral spirits and tack cloth……and repeat. Each time I have progressed to a finer grit of sandpaper. This is my 3rd round and I am still breaking through the paint to the primer (mostly at the edges) but I am okay with that because I figure each time my surface will get flatter and flatter. That is what I thought at least until I noticed that the surface that I am sanding isn't actually flat. It is actually slightly bowl shaped. The reason is that the rubber sanding block tends to flex down slightly at the ends which causes it to sand more at the front and back edges of each stroke. It isn't noticeable by eye but when I put a straight edge on it (spirit level) you can tell that the middle of the sanded piece is just slightly bowed outward. Is this a normal artifact of block sanding or do I need to adjust my technique?
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Old 27th May 2014, 04:20 AM   #2
puppet is online now puppet  United States
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If the block is what I'm thinking .. using it to flatten a surface will be futile. You need a flat board to establish "flatness". Flexible is great for curved panels ... ie autobody.

Another possibility is that the panel edges took on moisture ( it's springtime). Got to get control of that.

Might be your technique ... everybody tries to respect panel edges ... even subconsciously. I found it helpful to count while I was sanding. Make a mental picture of the sanding devises position on the panel. Regulate your arm pressure to provide equal weight distribution through the sanding pass. Each push and pull gets a count. When you change panel position use the same technique and count.

Hope this helps.
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Old 27th May 2014, 11:46 AM   #3
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Quote:
Originally Posted by puppet View Post
If the block is what I'm thinking .. using it to flatten a surface will be futile. You need a flat board to establish "flatness". Flexible is great for curved panels ... ie autobody.

Another possibility is that the panel edges took on moisture ( it's springtime). Got to get control of that.

Might be your technique ... everybody tries to respect panel edges ... even subconsciously. I found it helpful to count while I was sanding. Make a mental picture of the sanding devises position on the panel. Regulate your arm pressure to provide equal weight distribution through the sanding pass. Each push and pull gets a count. When you change panel position use the same technique and count.

Hope this helps.
That helps a lot! This is the block I am using:

3M Rubber Sanding Block - Power Sander Accessories - Amazon.com

I think I am confusing flatness with smoothness. I want the sandpaper to contact the entire surface as I sand without any low or high spots. If this is over a slight curve then I think that is okay because I will still be able to polish it to a mirror finish. I think I have almost removed all of the low and high spots. I am sanding in one direction rather than multiple directions. Once I get down to about 2000 grit sandpaper I will hit it with the polishing compound using a more random pattern to remove the sanding lines. Does this sound reasonable?
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Old 27th May 2014, 11:58 AM   #4
AndrewT is offline AndrewT  Scotland
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Yes, flat and smooth are quite different.

For a piano finish, you need flat and smooth. Equals a lot of work.

A stiffer rubbing block will help with flat.
I sometimes use a block of wood.
Size it to take a 1/3rd sheet of glasspaper.
Or use al oxide paper if you need a lot of correction to get to flat.
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Old 27th May 2014, 01:04 PM   #5
LFM is online now LFM  Canada
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Default blocking

You should try a thick piece of plywood as a block and hold it up to a straight edge to ensure it is flat. You should not apply much pressure and let the paper do the work. Clean the paper often. Try not to run more than a few mm past the edges. If you are still having trouble with the edges, clamp some stock level with the surface you are sanding so that you run off onto this instead of the cabinet edges. The paint will always be thinnest on the edges as the paint tends to run off the corners. I would recommend not getting too fussy with the color layer but buy some clear to go overtop and do most of the refinement in the clear. You can really load it on and even if it runs this can be sanded out. I would start with no more coarse paper than 600 grit and work up to 1500 or 2000. Finish with compound.
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Old 27th May 2014, 09:57 PM   #6
Bare is offline Bare  Canada
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All this with Rattle can Rustoleum.. Really !?
Sorry to tell you this but IMO: a Total waste of effort mate.
The paint itself is Low Quality .. intended to diy Paint one's rusting wheel barrow or Spade .. massively inappropriate when used as furniture coatings
Get some catalysed Urethane paint and have it applied by a Pro in a Real paintbooth ?
And even that assumes your wood work is stable.. as in it won't shrink and crack with the change of seasons.
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Old 27th May 2014, 10:20 PM   #7
Speakerholic
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Isn't a palm sander made for this?

EDIT: Never mind, I read your first post too quickly. Back into my hole.

You'll need a thicker paint than spray. If you can, do it Bare's way
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Old 27th May 2014, 10:24 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

You should sand at 90 degrees for each coat.
Doing it with topcoat back to primer is a poor
option compared to doing it mostly with primer.

High gloss and flat mirror finish are two different things.

If its not finished off with multiple layers of clear coat
then the paint finish will be very difficult to maintain.

Use leveller on each coat. (Simply solvent that
that lets the paint repeat the levelling process
of the top skin drying and shrinking first).

rgds, sreten.
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Old 27th May 2014, 11:21 PM   #9
evanc is online now evanc  United States
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Where in Jersey are you...I might be able to lend some assistance if you desire.
Evan
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Old 28th May 2014, 10:47 AM   #10
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So much work involved….ughhh.

I used rustoleum because it is a product that is easily available at my local hardware store and doesn't require any special equipment to apply. Getting a professional to do it for me kind of defeats the purpose of DIY…at least for me it does. When I show these off I want to be able to take credit for the finish as well as the sound.

I tried the rustoleum on a small piece of MDF before I even started spraying the speakers. Priming, painting, sanding down to 2000 grit, and hitting it with turtle wax rubbing and polishing compound produced an amazingly mirrored surface….perfectly reflective that I could have shaved with if I needed to. Applying that to larger surfaces seems to be the challenge.

I'm pretty sure that my problems getting everything flat started with the prep work. I should have gotten everything flat before I started painting…..it looked flat but obviously the sanding block is a much better judge of flatness than I am.

This is my first attempt at a finish like this so I guess it was inevitable that I would hit some….ahem….learning experiences as I progressed. I have two more speakers that I haven't even started painting yet so hopefully I will be able to apply all of this experience to finishing those.
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