I applied Minwax Wood Finish penetrating stain to white pine cabinet. Wiped stain thoroughly, waited 60 hours before attempting to apply Minwax water based, oil-modified polyurethane, using synthetic bristle brush. All of this is exactly as per instructions on products. Only thing I did that wasn't on Minwax instruction was to soak brush in paint thinner to release trapped air, then wipe on paper until dry before dipping in polyurethane. This advice was given by several different experts online. As I drew my brush across the piece, the poly immediately began beading up - took another stroke through... beading up through the entire length of the stroke. Immediately wiped the poly off and then wiped the area again using solvent. Piece may have lost a tiny bit of colour on that strip, but looks ok. What should I do now?
water based, oil-modified polyurethane
There's your problem right there. Paint thinner and waterbased finishes don't go together.
This advice was given by several different experts online.
That's in relation to standard varnishes (oil-based).
Never loading finish — this includes oil, varnish, paint or shellac — into a dry brush is sound practice, but you use the appropriate solvent in each case. With waterbased finishes that's water*.
*Usually plain water but some prefer water mixed with a little of an additive, e.g. flow aid or retarder.