Stop... take a breath.
Most finishing problems are procedural, and can be fixed with the
If you used an oil based stain, a couple of things probably happened.
(Disregard of you used a water borne color). The stain wasn't 100%
dry, even though it felt like that to you. When you put the poly on,
it dissolved the tiny amount of oil in the stain that had not dried
out. It brought the stain into the poly. When you sanded the poly,
you sanded off your color. It happens.
Solution: If you want to get the show on the road and are pressed for
time, stain your piece, the seal with dewaxed sanding sealer. Sand
lightly, apply poly according to instructions.
If you are not in a huge hurry, you should apply the (oil) stain to
the piece and let it sit for about a week. Then apply your poly as
Regardless of which avenue you choose, make sure your stain is put on
in light, even coats. If you need to apply more than one coat, wait
24 hours between coats so you can see what the wood will actually look
like with the stain. 2 - 3 light coats of stain are better than one
thick heavy one. This allow you to follow the amount of color
penetration in the different areas of your project and apply more
colorant as needed.
As a sidebar, I don't sand between coats of poly if I am actually
doing the finishing as a process. I recoat in 8 - 10 hours, and do
only a cursory sand for dust nibs. The poly will bite fine into the
But if I have to wait a day or two between coats, I hand sand with 220
to knock off the sheen. If it is a week before I can coat again, I
usually wait for about 10 day or more so I can sand more aggressively
before applying another coat.
Do not swirl or rub your sandpaper; follow the grain in long, even
strokes with the lightest pressure possible. Your finish is probably
showing marks due to incorrect technique. Use clean, new paper. When
it feels dull, you should have thrown it away a few minutes ago. If
the face of the paper clogs with finish, you are sanding too hard, or
using dull (or cheaply made) paper.
Sanding between coats is insurance for the finish to bite. A green
finish will be dry to the touch and probably won't mark, but it is
still quite soft. That's another reason for the marking. Make sure
you use a block with the paper wrapped around it. I am a big fan of
the foam sanding pads, and use them as sanding blocks for paper when
their own grit is gone.
Let us know how it goes.