You need to get it clean, dry, and oil-free. So I cleaned mine with soap and water
, then a solvent. A thinner compatible with your varnish
is good to use as a degreaser, then you'll know there'll be no incompatibility issues later.
Then I'm afraid you will have to sand it lightly to get a mechanical key for the varnish
to stick to. 320 grit is ideal.
Finally, remove dust with a tack cloth.
These days I get tack cloths and foam brushes
from amazon, in bulk.
I didn't like the finish from satin poly, too matt, so I went for spar varnish with a rubdown with rottenstone afterwards, to reduce the gloss. Admittedly, this is more work.