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I put 3 coats of spar on a wooden table. Unfortunately, there was a lot of dust particles on it as can bee seen on the lower piece in the attached image. So after doing some research I sanded it down with 400 grit sand paper. I did this by hand and tried to be careful not to take too much off. However, as you can see from the piece in the upper part of the image it has become dull with just a few shiny spots left. Reading many articles/posts and watching YouTube videos it appears to me that the shine shouldn't have gone away. I am saying this because everybody says you just sand it down a bit (some say 400 grit, others up to 2000 grit, others steelwool, etc) and then you buff it out with an automotive buffer. I didn't buy that yet but I can't image this will bring an even shine back on that upper piece. So what did I do wrong and where should I go from here?
 

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400 grit is too fine for what you have. Use 220 grit on a random orbital sander and apply another coat and see what happens. Chances are you will have to sand it again and apply yet another coat before you get the finish level. Before you apply another coat go to an automotive paint supplier and get some cone strainers and strain the varnish. There is likely dust in the can of finish you are using and the cone strainers paint stores sell are too coarse to remove it. Automotive strainers have a finer mesh.

In order to rub out a spar varnish and buff it it would be best if you allow the finish to dry a couple months. A spar varnish is deliberately made soft because it was designed for exterior wood. When you sand it, wet sand it with no coarser than 1200 grit paper and sand up to 2000 grit. Then use the auto polisher with a lambswool bonnet and rubbing compound.

In the future a spar varnish is a poor choice for interior furniture. Because it is made soft to deal with extreame wood movement it therefore doesn't hold up to daily abuse furniture gets as well as an interior varnish or polyurethane.
 

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Discussion Starter #3 (Edited)
Steve,

Thanks for the reply. So after I do the last coat and sand that with 1200+ sand paper, should the coating stay shiny or get dull again as in the picture? Or did I just not have enough coating and just stripped off most of it (it feel pretty smooth, though and I already did 3 coats but maybe too thin).

For the rubbing compound, would I use something like the JE Moser Rottenstone? I read some people are using that. Amazon J.E. Moser's 849839 Rottenstone

As for the strainer, the issue is actually that I have to work in the garage and because I leave the garage door open (to get good ventilation) I get dust on it since I live in a windy area. When I get some spar I poor some into a different container and then close the main container.

The table is actually an outside table. I started to get into a bad shape so I had to sand it quite a bit.

For the polisher I guess it would be something like this:
Amazon Black+Decker WP900 6" Random Orbit Waxer/Polisher
Amazon Cxtiy Polishing Buffer Pad Set Wool Plish for Car Glass Ceramic Stone
 

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I've never used that polishing compound so I can't say how good it is. I normally use 3M or Meguiar's rubbing compound.

If you have been brushing varnish and getting dust in the finish chances are you have gotten some of that dust back in the can. I just suggested straining the finish to be sure what you use in the future was clean. Then sometimes it's not dust. Sometimes you can buy a new can of varnish that has sat on a store shelf so long it starts making tiny chunks of dried varnish in the can. From the picture this is really what appears to be the case. Since you are fighting to make the finish smooth better to be safe. I even use this procedure to polish out cloudy plastic headlights on a car.

The polishers you show on the links will work however may be more labor intensive. They are more for a finish that has already been buffed and you use that to bring back the luster. I use one like this. https://www.harborfreight.com/7-in-10-amp-variable-speed-polisher-60626.html When you sand a finish it's like you sanded a clear piece of plexiglass. The scratches made by the sandpaper will completely take the shine off and make the surface cloudy. After sanding from 1200 up to 2000 grit you use the auto polisher and rubbing compound to buff off the scratches made by the sandpaper.
 
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