That ripple is what you're looking to sand out when you go for flat. You
can feel them if you run your fingers very lightly down the surface slowly.
Over time you develop a good feel for finding them. Do a little autobody
repair and you'll quickly become an expert at finding them. Either you do,
or the word "dambkit" becomes a frequent part of your vocabulary - usually
just after the clear coat goes on.
Grab yourself a paint stick and wrap it with your sandpaper (wrap the whole
stick) and lay it flat on your surface and sand. It will flatten those
ripples out nicely. Don't stop until the entire surface of the piece shows
sanding marks evenly. Get it back up to the desired level of shine after
that and then stand back and look at it at that shallow angle. You'll love
what you see. For the type of ripple you're describing I probably would not
go more aggressive than 1000 grit. You could go to 600 but you'd want to be
quite careful. 600 can take finish down pretty quickly. 1000 will take
more work but it will get there and there's less chance of burning through
if you're not really accustomed to this stuff. Either way it's work. If
you start at 600 your going to go through more stages to be rid of the
sanding marks. If you start at 1000 you're going to sand more to get to the
same level, but you'll only have one step before buffing. If you're really
into it you can buff from 1000 but I prefer to sand more and buff less. I
just hate buffing.
The real answer is to get it to the finish you like and stop there. No
point in getting anal about it. So - if you like it where it is then ignore
my comments and give yourself a pat on the back. Seriously - I'm not being
a wise guy. If however you want to dabble with it more or if you want
something to look at in the future, then grab that paint stick as I
suggested before and wrap it up and lay it down on the project and do a
little sanding. If you have low spots, they'll show up immediately as
unsanded areas. If you see nice even sanding marks on the entire surface
then you're flat. But like I said, the finish you strive for is the finish
that makes you happy. It is after all, all about pleasure.
You can also use something larger than a paint stick to take down a bigger
area at a time. The paint stick was just a common thing that I suggested.
I really would not go any coarser than the 400 you're using though. It will
be difficult to get the scratches out before you run out of finish. I'd try
to stay at and above 600 even if it takes longer.
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